Abstract
The utility of a dense subgraph in gaining a better understanding of a graph has been formalised in numerous ways, each striking a different balance between approximating actual interestingness and computational efficiency. A difficulty in making this tradeoff is that, while computational cost of an algorithm is relatively welldefined, a pattern’s interestingness is fundamentally subjective. This means that this latter aspect is often treated only informally or neglected, and instead some form of density is used as a proxy. We resolve this difficulty by formalising what makes a dense subgraph pattern interesting to a given user. Unsurprisingly, the resulting measure is dependent on the prior beliefs of the user about the graph. For concreteness, in this paper we consider two cases: one case where the user only has a belief about the overall density of the graph, and another case where the user has prior beliefs about the degrees of the vertices. Furthermore, we illustrate how the resulting interestingness measure is different from previous proposals. We also propose effective exact and approximate algorithms for mining the most interesting dense subgraph according to the proposed measure. Usefully, the proposed interestingness measure and approach lend themselves well to iterative dense subgraph discovery. Contrary to most existing approaches, our method naturally allows subsequently found patterns to be overlapping. The empirical evaluation highlights the properties of the new interestingness measure given different prior belief sets, and our approach’s ability to find interesting subgraphs that other methods are unable to find.
Keywords
Dense subgraph patterns Community detection Subjective interestingness Maximum entropy1 Introduction
Mining dense subgraph patterns in a given graph is a problem of growing importance, owing to the increased availability and importance of social networks between people, computer networks such as the internet, relations between information sources such as the world wide web, similarity networks between consumer products, and so on. Graphs representing this type of data often contain information in the form of specific subsets of vertices that are more closely related than other randomly selected subsets of vertices would be.
For example: a dense subgraph pattern in a social network could represent a group of people with similar interests or involved in joint activities; a dense subgraph pattern on the world wide web could represent a set of documents about a common theme; and a dense subgraph pattern in a product copurchasing network (in which products are connected by an edge if they are frequently bought together) could represent a coherent product group.
A multitude of methods have been proposed for the purpose of discovering dense subgraph patterns, most of which belong to one of three categories. The first category starts from the full graph, and attempts to partition it (typically in a recursive way) such that each block in the partition is in some sense densely connected while vertices coming from different blocks tend to be less frequently connected. The second category generalizes the notion of a clique, e.g. to sets of vertices between which only a small number of edges are absent. The third category attempts to fit a probabilistic model to the graph. This model is typically such that vertices belonging to the same ‘community’ (which forms a dense subgraph) are more likely to be connected.
Despite these differences, all approaches for dense subgraph mining are similar in implicitly or explicitly assuming a measure of interestingness for dense subgraph patterns, to be optimised by the dense subgraph mining algorithm. The interestingness measure used essentially affects two aspects of the dense subgraph mining process: the computational cost of finding the most interesting dense subgraphs, and the degree to which presenting this pattern helps the user to increase their understanding about the graph.
As such, the design of a dense subgraph mining method has been approached very much as an engineering problem, tradingoff conflicting requirements. This approach has long seemed acceptable (and even inevitable) given that true interestingness of a dense subgraph pattern eludes objective formalisation anyway, as it is fundamentally subjective: interestingness can only be defined against the background of prior beliefs the user already holds about the graph. For example, it will be less of a surprise to a user to hear that a set of vertices believed to all have a high degree form a dense subgraph, than that an equally large set of supposedly lowdegree vertices form a dense subgraph, and thus the latter is subjectively more interesting to that user.
Because of this, the most basic question: “How interesting is a given dense subgraph pattern to a given user?” has evaded rigorous scrutiny. Previous research does not shine much light on what this interestingness looks like, on whether any of the engineered interestingness measures approximate it well, and on whether it can be optimised efficiently. Yet, recent results on the formalisation of subjective interestingness and its applications to other exploratory data mining problems (De Bie 2011a, b) has made clear that this question is actually wellposed.
After formalizing subjective interestingness, we make it clear how the resulting measures are different from previous proposals, holding the middle between measures based on absolute missing edge tolerance and measures based on relative missing edge tolerance (Sect. 4). Furthermore we propose two effective algorithms for finding the most interesting (set of) dense subgraph patterns (Sect. 3), one of which is a fast heuristic and the other exact and hence necessarily slower.^{1} Our empirical results illustrate the effectiveness of the search strategies, how the results are (usefully) different from those of a stateoftheart algorithm for mining dense subgraph patterns, how different prior beliefs matter in the determination of subjective interestingness, and how the proposed algorithms perform computationally (Sect. 5).
2 Subjective interestingness of dense subgraph patterns
2.1 Notation
A graph is denoted \(G=(V,E)\), where V is a set of n vertices (usually indexed using a symbol u or v) and \(E\subseteq V\times V\) is the set of edges. The adjacency matrix for the graph is denoted as \(\mathbf {A}\), with \(a_{u,v}\) equal to 1 if there is an edge connecting vertices u and v, and 0 otherwise.
For the sake of simplicity, we focus the exposition on undirected graphs without selfedges in this paper, for which it holds that \((u,v)\in E \Leftrightarrow (v,u)\in E\) and \((u,v)\in E\Rightarrow u\ne v\). However, most of our results immediately apply also to directed graphs or graphs that allow selfedges. We will briefly outline how in Sect. 2.3.1.
The setup in this paper is that the user knows (or has direct access to) the list of vertices V in the graph, and their interest is in improving their understanding of the edge set E. Thus, the data to be mined is the edge set E, and the data domain is \(V\times V\) (with the additional constraints for undirected graphs without selfloops).
2.2 Formalising dense subgraph patterns
The term ‘pattern’ has been overloaded numerous times in the wider data mining literature, so it is important to make it clear exactly what is meant by this term in the current paper. We adhere to the definition adopted in the general framework introduced by De Bie (2011a). There, a pattern is any piece of information about the data that limits its set of possible values to a subset of the data domain. In the present context, a pattern is any piece of information about the graph that limits the possible values of the edge set E to a subset of the data domain \(V\times V\). Note that this setup naturally accommodates iterative data mining: in each iteration the domain is further reduced by the newly presented pattern.
As the focus of the paper is on dense subgraph patterns, the kind of patterns we will use informs the user that the density of a specified vertexinduced subgraph is equal to or larger than a specified value. A pattern of this syntax can be uniquely specified by means of a pair \((W,k_W)\), where \(W\subseteq V\) is the set of vertices in the subgraph and \(k_W\) is a lower bound on the number of possible edges between these vertices that are actually present in the graph G. By \(n_W\) we will denote the number of possible edges between vertices from W, equal to \(\frac{1}{2}W(W1)\) for undirected graphs without selfedges.
Continuing our example in Fig. 1, the orange, dashed pattern can be specified as \((\{1,2,3,4,5\},8)\), meaning that at least \(k_W=8\) edges exist between the vertices from \(W=\{1,2,3,4,5\}\). The number of possible edges, \(n_W\), equals 10, since \(W = 5\).
2.3 A subjective interestingness measure
Many authors have previously attempted to quantify the interestingness of dense subgraph patterns in objective ways (see Sect. 4). Each of these attempts is based on the intuition that a subgraph is more interesting if it covers more vertices, and if only few pairs of these vertices are not connected. However, they differ in how to quantify the number of missing edges (e.g. in a relative or in an absolute manner), and in how to tradeoff these two aspects.
A general framework for formalising subjective interestingnessIn this paper we make no attempt at proposing an objective interestingness measure. Instead we use the framework proposed by De Bie (2011a, b), which lays out general principles for how to quantify the interestingness of data mining patterns in a subjective manner. This is done by formalising the interestingness of a pattern with respect to a socalled background distribution P for the data, which represents the belief state of the user about the data. More specifically, the background distribution assigns a probability to each possible value of the data according to how plausible the user deems it to be.

The information content of the pattern, which is the negative log probability that the pattern is present in the data, computed using the background distribution.

The description length of the pattern, i.e. the length of the description needed to communicate the pattern to the user.
The centrality of the evolving background distribution in this framework ensures that it naturally captures the iterative nature of the exploratory data mining process. Indeed, upon observation of a pattern, the user’s beliefs will include the newfound knowledge of this pattern, resulting in a change in the background distribution. This update to the background distribution reflects the fact that the observation of a pattern may affect the subjective interestingness of other patterns (indeed, some patterns make others more or less plausible). Then the most interesting pattern with respect to the updated background distribution \(P'\) can be found, and the process can be iterated.
To use this framework, we need to understand how to formalise prior beliefs at the start of the mining process in an initial background distribution P, and how it evolves upon presentation with a pattern. It was argued the maximum entropy distribution subject to the prior beliefs as constraints is a good choice for the initial background distribution. For the evolution upon presentation with a pattern, it was argued that the background distribution should be conditioned on the presence of the pattern (De Bie 2011a).
Applying the framework to dense subgraph patterns While this abstract framework is generally applicable at least in principle, how it is deployed for specific prior beliefs, data, and pattern types, is often nontrivial. The first main contribution of this paper is to do this for the important case of dense subgraph patterns in a graph.
For dense subgraph patterns, the data consists of the edge set \(E\subseteq V\times V\), and the patterns are of the form specified in Sect. 2.2. Thus in the present section we will discuss the kinds of initial prior beliefs for such data that we will consider in this paper, and what the resulting background distribution is (Sect. 2.3.1); how the background distribution evolves upon presentation with a pattern (Sect. 2.3.2); how to compute the information content of the patterns we consider (Sect. 2.3.3); how to compute their description lengths (Sect. 2.3.4); and finally how the information content and description length are combined to yield the subjective interestingness measure proposed in this paper (Sect. 2.3.5).
2.3.1 The initial background distribution
 (1)Prior beliefs on individual vertex degrees In the more complex case, the user holds prior beliefs about the degree of each of the vertices in the graph. De Bie (2011b) showed that the maximum entropy distribution then becomes a product of independent Bernoulli distributions, one for each of the random variables \(a_{u,v}\), defined to be equal to 1 if \((u,v)\in E\) and 0 otherwise. More specifically, it is of the form:where Z is a normalisation constant (the ‘partition function’) equal to \(Z=\prod _{u<v}\left( 1+\exp ((\lambda _u+\lambda _v)\right) \), so that:$$\begin{aligned} P(E)= & {} \frac{1}{Z}\prod _{u<v} \exp ((\lambda _u+\lambda _v)\cdot a_{u,v}), \end{aligned}$$As a product of Bernoulli distributions, this distribution can conveniently be represented by a matrix \(\mathbf {P}\in [0,1]^{n\times n}\), where the rows and columns are indexed by the vertices, and where \(p_{u,v}= \frac{\exp (\lambda _u+\lambda _v)}{1+\exp (\lambda _u+\lambda _v)}\) denotes the probability that \(a_{u,v}=1\), i.e. that there is an edge between vertices u and v (note that for undirected graphs without selfloops \(\mathbf {P}\) is symmetric and has zeros on the diagonal).^{2} The parameters \(\lambda _u\) and \(\lambda _v\) thus directly determine the probability \(p_{u,v}\) for the edge between vertices u and v: the larger \(\lambda _u\) and \(\lambda _v\), the larger this probability. Given the assumed degrees for the vertices as specified by the prior beliefs, inferring the value of these parameters \(\lambda _u\) is a convex optimisation problem, and the algorithm presented by De Bie (2011b) for doing that easily scales to millions of vertices.$$\begin{aligned} P(E)= & {} \prod _{u<v} \frac{\exp ((\lambda _u+\lambda _v)\cdot a_{u,v})}{1+\exp (\lambda _u+\lambda _v)}. \end{aligned}$$
 (2)
Prior belief on the overall graph density In the more simple use case we consider here, the user only has a prior belief about the overall density of the graph (or equivalently, on the average vertex degree). It is easy to show that the maximum entropy distribution subject to this prior belief is also a product of Bernoulli distributions, but now with all entries \(p_{u,v}\) from \(\mathbf {P}\) equal to the assumed (relative) edge density. Thus, also in this use case the background distribution is a product distribution with a factor for each vertex pair, fully parameterised by a matrix \(\mathbf {P}\).
The number of prior belief types of possible interest is clearly unbounded, and the purpose of the paper is by no means to be comprehensive in this regard. Let us just note that although the computational cost of the algorithms will vary depending on the kinds of prior beliefs considered, the general approach outlined below is not specific for any kind of prior belief type.
2.3.2 Updating the background distribution throughout the mining process
Upon presentation of a pattern, the user’s belief state will evolve to become consistent with this newly acquired knowledge, which should be reflected in an update to the background distribution. More specifically, this updated background distribution \(P'\) should be such that the probability that the data does not contain the pattern is zero. To see what this means in the present context, let us introduce the function \(\phi _W\), which counts the number of edges within the vertexinduced subgraph induced by \(W\subseteq V\), i.e. \(\phi _W(E)=\sum _{u,v\in W, u<v} a_{u,v}\). Then, following the presentation of a pattern \((W,k_W)\) to the user, \(P'\) should be such that \(\phi _W(E)\ge k_W\) holds with probability one. Let us denote this set of consistent distributions as \(\mathcal {P}'\).
Theorem 1
The proof is given in the “Appendix”.
Corollary 1
Using the same notation as in Theorem 1, and for \(u,v\in W\), it holds that \(\log \left( \frac{p'_{u,v}}{1p'_{u,v}}\right) =\log \left( \frac{p_{u,v}}{1p_{u,v}}\right) +\lambda _W\). I.e., the effect of updating the background distribution is that the logodds of an edge between any pair of vertices \(u,v\in W\) is increased by \(\lambda _W\).
As the updated background distribution is again a product of independent Bernoulli distributions, the process of updating the background distribution can be iterated by repeatedly invoking the theorem. In each iteration, upon presentation of a pattern \((W,k_{W})\) a new variable \(\lambda _{W}\) would be introduced, which affects the probabilities of edges connecting vertices within W in such a way that their logodds are increased by \(\lambda _W\). This is precisely how the background distribution is updated in the experiments below.
Remark 1
It would be inefficient to store the updated edge probabilities at each iteration of the mining process, as their number is quadratic in the number of vertices. Instead, it is much more efficient in practice to only store the \(\lambda _W\) variables, and to compute the probabilities from these as and when needed.
This can be done by exploiting Corollary 1, which implies that the logodds of the probability of an edge between a pair of vertices \(u,v\in V\) is equal to the logodds of this probability under the initial background distribution, plus the sum of the \(\lambda _W\) variables corresponding to all patterns \((W,k_{W})\) for which \(u,v\in W\).
The logodds under the initial background distribution with prior beliefs on individual vertex degrees is equal to \(\lambda _u+\lambda _v\) for the vertex pair (u, v), and hence it can be computed in constant time by storing only V parameters. For the initial background based on a prior belief on overall density, the logodds is a constant.
After showing the user a series of patterns \((W,k_{W})\), the odds for an edge between u and v will have become \(\lambda _u+\lambda _v+\sum _{W:u,v\in W}\lambda _{W}\) under the updated background distribution. This corresponds to an edge probability equal to \(\frac{\exp (\lambda _u+\lambda _v+\sum _{W:u,v\in W}\lambda _{W})}{1+\exp (\lambda _u+\lambda _v+\sum _{W:u,v\in W}\lambda _{W})}\).
Remark 2
Note that after updating, the constraints on the expected degrees of the vertices used in fitting the initial background distribution may no longer be satisfied. This should not be surprising and is in fact desirable, as the initial constraints merely reflect initial beliefs of the user. These beliefs can be incorrect or inaccurate, and will evolve after observing a pattern.
On the other hand, any constraint imposed by the observation of a pattern will remain satisfied throughout subsequent iterations in the mining process. This follows from the fact that \(\lambda _W\ge 0\), such that \(p'_{u,v}\ge p_{u,v}\): the individual edge presence probabilities can only increase after updating a background distribution at any stage in the mining process. Thus, the expected value of the functions \(\phi _W(E)\) can only increase, such that if \(\sum P'(E) \phi _W(E)\ge k\) following an iteration of the mining process, this inequality will continue to hold in later iterations.
2.3.3 The information content
The information content is the negative log probability of the pattern being present under the background distribution. Thus, to compute it we need to be able to compute the probability of a pattern under the background distribution. Here we will show how this can be done, exploiting the fact that from Sects. 2.3.1 and 2.3.2 we know that the initial as well as the updated background distributions considered in this paper are products of Bernoulli distributions. This means that the background distribution can always be represented by means of a matrix \(\mathbf {P}\) as detailed in Sect. 2.3.1.
Given a pattern \((W,k_W)\) and a background distribution defined by \(\mathbf {P}\), the probability of the presence of the pattern is the probability that the number of successes in \(n_W\) Bernoulli trials with possibly different success probabilities \(p_{u,v}\) is at least equal to \(k_W\). This can be computed reasonably (though not very) efficiently using the Binomial distribution as long as the background distribution is constant, i.e. \(p_{u,v}=p\) for all \((u,v)\in E\) (i.e. for all possible edges). It is harder if the background distribution is not constant though.
Fortunately, we can tightly upper bound this probability by means of the general Chernoff/Hoeffding bound (Chernoff 1952; Hoeffding 1963):
Theorem 2
Here, \(\mathbf {KL}\left( \hat{p} \Vert p \right) \) is the KullbackLeibler divergence between two Bernoulli distributions with success probabilities \(\hat{p}\) and p respectively, i.e. \(\mathbf {KL}\left( \hat{p} \Vert p \right) =\hat{p}\log \left( \frac{\hat{p}}{p}\right) +(1\hat{p})\log \left( \frac{1\hat{p}}{1p}\right) \).
2.3.4 The description length
2.3.5 The subjective interestingness
2.3.6 A detailed example of subjective interestingness
The subjective interestingness that we just formalised, including the two cases of prior beliefs, was also used to obtain the example shown in Fig. 1. In particular, the orange, dashed subgraph is the pattern having the highest Interestingness when considering graph density as prior belief, and the purple, dotted subgraph is the pattern having the highest Interestingness when considering individual vertex degrees as prior belief. In both cases, \(q = 0.2\) was used; the effect of q is negligible for large networks, but a higher and more realistic value for q is required to obtain reasonable results on smaller graphs. Here, q can be loosely interpreted as the ‘expected probability’ for a random vertex to be part of a dense subgraph pattern.
When comparing the two most interesting patterns, it is immediately obvious that they are quite different. In fact, they are in different parts of the graph and their intersection is empty. When one only knows the average degree of all vertices, any high density subgraph is deemed interesting, as is common in most existing approaches to dense subgraph mining (although our formalisation of ‘density’ is different, see Sect. 4.3.1). With our approach, however, it is also possible to inject other prior knowledge and use this to make interestingness subjective. This is the key to the iterative mining scheme presented in Sect. 2.3.2, but also other types of prior beliefs can be considered. The case we consider in this paper is prior beliefs on the individual vertex degrees, which generally results in the discovery of smaller and sparser subgraphs that are nevertheless surprisingly dense considering the degrees of their individual vertices.
3 Algorithms
In this paper, our focus is on the interestingness measure and, more specifically, on formalising subjective interestingness. Because our interestingness measure is more complex than measures based on density only, the search for the most interesting dense subgraph pattern cannot be expected to be as efficient. The search is challenging indeed, but we nonetheless develop two practically scalable algorithms to do this. The second main contribution of this paper is the introduction of two algorithms for finding dense subgraph patterns in a graph. One uses a heuristic search strategy for maximum scalability, the other uses an exact search strategy for maximum accuracy.
3.1 Heuristic search
The algorithm requires the recursive computation of the interestingness measure, and thus of \(k_{W'}\) and \(p_{W'}\) for \(W'=W\cup \{v\}\) or \(W'=W{\setminus }\{v\}\). Based on the values of \(k_W\) and \(p_W\) this can be done efficiently in O(W) time. Using these two quantities, computing \(\text{ Interestingness }(W',k_{W'})\) can then be done in constant time. For improved efficiency, we only consider expansions that keep the subgraph connected.
 All

Each of the separate vertices forms a seed, i.e. \(\{v \mid v \in V\}\).
 Uniform( k )

A selection of k of the vertices separately, selected uniformly at random from V but without duplicates.
 TopK( k )

The topk vertices, separately, with respect to the interestingness of their corresponding neighbourhoodinduced subgraphs (i.e. the vertex itself along with all its direct neighbours in the graph).
3.2 Exact search
On moderately sized graphs, exact search may be feasible. Besides being useful in its own right in such applications, comparing the results of the hillclimber with the results of an exact search algorithm on smaller data will give insight into the effectiveness of the hillclimber. Thus, we develop an exact bestfirst search strategy that is similar to the A* algorithm. This algorithm is investigated only for the constant background distribution, as that allows us to use discrete data structures that lead to a particularly efficient implementation.
Typically we are only interested in the most interesting pattern, possibly to be iterated after updating the background distribution if more than one pattern is desired. Hence, we could use an A*type of algorithm if an optimistic estimate can be made, i.e. if an upper bound on the interestingness that any supergraph of a given subgraph pattern can achieve can be computed.
Given such an optimistic estimate, the A*type algorithm maintains a priority queue of candidate subgraphs sorted in order of decreasing value of the optimistic estimate. Then, the first pattern from the priority queue is iteratively selected and for each vertex not yet part of it a new pattern is created by adding it to the pattern. The pattern is then removed and the expanded candidate patterns are inserted in the priority queue. This iterative process is repeated until the optimistic estimate of the firstranked pattern is lower than the actual interestingness of the best pattern found so far.
While this can be done in general, for simplicity and speed, we develop it only for the case of a constant background distribution. This allows us to use discrete data structures and hence greater efficiency. In this case, \(p_W\) is independent of W and equal to the assumed edge density of the graph. Consequently, the interestingness for any expanded subgraph \(W'\supseteq W\) only depends on \(n_{W'}\) and \(k_{W'}\).
 1.
Edges connecting two vertices from W.
 2.
Edges connecting a vertex from \(W'{\setminus }W\) with a vertex from W.
 3.
Edges connecting two vertices from \(W'{\setminus }W\).
To compute a bound on the number of vertices of the third kind, we need for each vertex in \(V{\setminus }W\) the degree within the subgraph induced by the vertices \(V{\setminus }W\). Again, this set of values can be computed very efficiently using fast set intersection operations. Then sum of the largest \(W'{\setminus }W\) such values, each thresholded at \(W'{\setminus }W\)1 (since this is the maximum number of neighbours there can be within \(W'{\setminus }W\)), is a bound on the number of vertices of the third kind. Adding the (bounds on) the number of edges of each of these three kinds yields an upper bound on \(k_{W'}\), and thus on the interestingness of \(W'\) given its size. The overall upper bound can be found by computing the largest upper bound for all possible sizes of \(W'\). This can be efficiently done in a forloop from W to V, iteratively computing an upper bound for each consecutive \(W < W' \le V\) and taking the maximum as global optimistic estimate. This loop can be broken as soon as there are no more edges of the second or third kind left that can be added.
Although this bound could be further tightened and developed also for the prior belief using individual degrees, this would come at the expense of additional computational cost. We therefore leave a thorough investigation of this topic for future work. As the empirical evaluation will demonstrate, the presented estimate is sufficiently tight to allow us to achieve our main goal: providing a reasonably fast baseline to compare the quality of the hillclimber’s results to, on a number of moderately sized graphs.
4 Discussion and related work
Our contributions are related to three different areas of research: the development of subjective interestingness measures in data mining; the development of instant and interactive methods for pattern mining; and the wider literature on dense subgraph mining. Here we discuss some insightful connections to each of these.
4.1 Subjective interestingness in data mining
The data mining literature, and the local pattern mining literature in particular, abounds with papers on the formalisation of interestingness for various kinds of patterns (see e.g. McGarry 2005; Geng and Hamilton 2006 for two surveys on the topic). Part of that work is focused on subjective interestingness measures, which are often conceived as measures that quantify the amount of ‘novelty’ or ‘surprise’ a pattern presents to the user. A recent survey on this topic (Kontonasios et al. 2012) distinguishes two main classes of approaches: the syntactic approaches, which often work by encoding the prior knowledge about the data in a set of rules, patterns, a taxonomy, or ontology; and the probabilistic approaches, which often represent the user’s knowledge about the data using a probability distribution of the data (specified explicitly or implicitly).
The generic approach from De Bie (2011a), on which the contributions in the present paper are built, belongs to the category of probabilistic approaches, and is most similar in spirit to the swap randomisation approach from Gionis et al. (2007), Hanhijarvi et al. (2009). The swap randomisation approach aims to capture the prior beliefs of the user in the form of a set of constraints, similar to De Bie (2011a). However, it does not attempt to represent the belief state of the user in the form of an explicitly represented background distribution. Instead, it is based on the ability to directly sample randomised versions of the data while maintaining the prior belief constraints satisfied, bypassing the need for the background distribution. These randomised data samples then allow one to compute an empirical p value for any given pattern, quantifying the amount of surprise it presents to the user, and hence its subjective interestingness.
There are a number of important advantages to the approach advocated in De Bie (2011a) though, related to the fact that having access to the explicit background distribution allows one to compute the interestingness analytically. This is crucially important, as the most interesting patterns will tend to have a very small p value, such that discerning between them using a swap randomisation approach would require an unrealistically large number of randomised data samples to be drawn. Second, it would be infeasible to mine the most interesting patterns directly using a swap randomisation approach, as it would require running the costly randomisation procedure at each step during the search process. With an analytically computable interestingness measure, however, this is feasible as demonstrated in the present paper.
Finally, we note that a comment often heard about subjective measures of interestingness is that they become objective measures as soon as the prior beliefs or background knowledge is fixed. This is of course the case: once the user is fixed, the subjectiveness is factored out and the interestingness is fully determined in principle. However, the particular aspect of an interestingness measure that makes it subjective is that this dependency on the user is made explicit by treating the user as a variable input to the interestingness function (see also Kontonasios et al. 2012), such that, at least in principle, it is possible to quantify interestingness for other users as well. It is to make this clear that in the present paper we considered two kinds of prior beliefs, rather than just one.
4.2 Instant and interactive pattern mining
A recent trend in the literature is the development of instant and interactive pattern mining techniques. van Leeuwen (2014) provides a recent overview, including open challenges for future research. Contributions in this area can be roughly classified into three categories.
The first category concerns pattern sampling algorithms, often also called output space sampling to emphasize the difference from data sampling; the latter simply reduces the size of the problem to reduce its complexity, whereas the former considers the complete problem but only returns a sample from the full solution set. Hasan and Zaki (2009) introduced a sampling framework based on the Metropolis–Hasting algorithm to sample from the output space of all frequent subgraphs, and showed that frequent patterns can be sampled, e.g., uniformly or proportional to support. Boley et al. (2011) presented a direct, hence more efficient sampling procedure for itemsets. Although these methods can be used to obtain small numbers of patterns, the patterns (1) can only be sampled according to some objective interestingness measure and (2) iterative mining, where each new result is interesting relative to all its predecessors, is currently not possible.
The second category concerns interactive mining algorithms that aim to infer some kind of subjective interestingness from user feedback. The first work in this direction, by Bhuiyan et al. (2012), proposed to use user feedback to adapt the sampling distribution of itemsets, and is therefore also closely related to the methods in the first category. More precisely, it performs Markov Chain Monte Carlo (MCMC) sampling of frequent patterns and the user is allowed to provide feedback by liking or disliking them. This feedback is used to update the sampling distribution, so that new patterns are mined from the updated distribution.
In similar spirit, Dzyuba and van Leeuwen (2013) proposed Interactive Diverse Subgroup Discovery (IDSD), an interactive algorithm that allows a user to provide feedback with respect to provisional results and steer the search away from regions that she finds uninteresting. Later, Boley et al. (2013) and Dzyuba et al. (2014) simultaneously (and independently) developed methods to learn pattern rankings using techniques from preference learning. Boley et al. also presented a working system for what they called ‘oneclickmining’, in which the preferences of the user for certain algorithms and patterns are learned. Nevertheless, only objective interestingness measures are used to mine patterns, which are then presented to the user. For each of these methods, prior beliefs and/or mined patterns can not be used to explicitly adapt interestingness, and iterative mining of a nonredundant set of interesting patterns is not possible.
The third and final category concerns working pattern mining systems / tools with a graphical user interface, that have been developed with a focus on instant and interactive use. A prime example is MIME (Goethals et al. 2011), for mining and browsing (frequent) itemsets according to a number of objective interestingness measures. These systems, however, first mine a (large) number of patterns and then give the user the opportunity to browse this collection; subjective interestingness and interative mining are not supported.
4.3 Dense subgraph mining
We now survey the most prominent and most directly related work on the topic of dense subgraph mining.
4.3.1 Structural measures
Unfortunately, in most applications each of these measures exhibits a bias that makes it practically hard to use. For example, the relative edge density is easily maximised and made equal to 1 simply by considering very small subgraphs (e.g. containing 2 vertices connected by 1 edge). On the other hand, the average degree tends to be (trivially) large for large subgraphs, simply because there are so many vertices any vertex can possibly be connected to. Similarly, it is usually easy to find large kcores, whereas it is trivially easy to find very small kplexes. The edge surplus, in being an absolute difference between two quantities that grow with the size of the subgraph, tends to be larger for larger subgraphs simply by virtue of being larger.
Yet, an advantage of all these measures of interestingness is their transparency: it is easy to explain what they mean. However, although our proposed measure is relatively efficient to compute, at first sight its relation with these objective structural interestingness measures is less obvious.
The numerator in this approximating upper bound makes it clear that the proposed interestingness measure is similar to the edge surplus when \(\beta \) is large relative to \(\alpha \) (such that the denominator is approximately constant). A key difference though is that the expected number of edges in our measure is computed as \(p_Wn_W\), i.e. with respect to the background distribution (rather than being determined by a parameter \(\gamma \), the value of which is not related to prior beliefs or any other relevant information). Thus, this probability itself varies with the subgraph W considered. However, even ignoring this difference, the upper bound on the proposed interestingness measure differs from the edge surplus by a factor equal to an affine function of the number of vertices in the subgraph. This difference is desirable as further supported by the arguments below in Sect. 4.3.3.
The denominator normalises the edge surplus, and for \(\beta \) small relative to \(\alpha \) it makes this bound very similar to what could be called the average degree surplus, which for vanishing \(p_W\) would become equal to the average degree.
Thus, (the upper bound on) our proposed interestingness measure combines elements from a number of objective interestingness measures, in addition to providing a means of injecting prior beliefs. This connection to previously proposed measures, resolving the issues they individually suffer from, strongly corroborates the principles from De Bie (2011a) used to derive this interestingness measure.
4.3.2 Newman’s modularity measure
A problem shared by the structural measures listed above is that the subgraph patterns they reveal are often the result of common knowledge or statistically trivial information. Our proposed measure solves this issue by taking prior beliefs into account, and by assigning a high interestingness value only if the pattern is surprising against that background. To some extent, this idea also underlies the measure of modularity, which was proposed to evaluate the quality of a partition of a network into (nonoverlapping) communities (Newman and Girvan 2004). Modularity is equal to the difference between the number of edges within the partitions and the expected number of edges based on the configuration model (random graph model with specific degree sequence).
However, our method is different to modularity as it quantifies interestingness of individual overlapping subgraphs and is not bound to a specific background distribution. Additionally, modularity is essentially an absolute measure (being equal to the difference between actual number of edges and expected number of edges), and as a result it has been found to prefer large subgraphs (sometimes even if these consist of two smaller subgraphs connected by just a single edge) (Fortunato and Barthelemy 2007). And finally, it is not designed to handle overlap between subgraph patterns as it is essentially evaluating the global partition of the data rather than the quality of a single subgroup individually.
4.3.3 Hypothesistesting based measures
Our chosen pattern syntax is such that the probability of the pattern being present is directly equivalent to a p value. Here, the null hypothesis is represented by the background distribution P, and the test statistic is equal to the number \(k_W\) of edges connecting the vertices from the set W for the pattern considered. With this null hypothesis and test statistic, the p value would be equal to the probability to observe \(k_W\) or more edges connecting vertices from W in data sampled from P, which is precisely the probability of the pattern \((W,k_W)\). That means that the information content is logarithmically related to the weight of evidence (as quantified by the p value) the pattern provides against the background distribution. This is directly in line with approaches that advocate the use of (empirical) p values to rank patterns, such as the approaches based on swap randomisation (Gionis et al. 2007; Hanhijarvi et al. 2009). An important advantage of our approach is however that the p values are computed analytically, which means that they are more accurate, and more importantly, that we can use them dynamically during search, and this without expensive computations. (Note that it was already pointed out in De Bie (2011a) that p values are indeed a special case of the information content for particular types of patterns.)
Additionally, our approach trades off this p value (i.e. information content) with the description length of the pattern. This means that the most interesting pattern is not necessarily the most surprising one in the sense of the p value. There are good reasons for this in addition to the motivations in De Bie (2011a), related to the multiple hypothesis testing issue. Indeed, the more hypothesis tests are being considered, the higher the probability that one of them turns out to be significant by chance. Normalising with the description length is similar in spirit to a multiple testing correction, demanding a more significant p value for larger patterns to account for their higher complexity. As such the multiple testing effect is controlled, making it less likely that the most ‘interesting’ pattern is actually a fluke.^{9}
5 Experiments
In this section we will use the acronym SSGc, for ‘Subjective SubGraph  constant’, to refer to our approach with a prior belief on the overall number of edges, and SSGi, for ‘Subjective SubGraph  individual’, to refer to the background distribution incorporating prior beliefs on the degree of each individual vertex. In all experiments, q is set to \(q=0.01\). While not reported here, we observed that the results are very robust w.r.t. the choice of this parameter, especially for larger datasets.
For each network, given are its data source, the number of vertices, the number of edges, and its edge density
Source  Dataset  V  E  Density 

Newman  Karate  34  78  0.139 
Newman  Dolphins  62  159  0.084 
Newman  Lesmis  77  254  0.087 
Newman  Polbooks  105  441  0.081 
Newman  Adjnoun  112  425  0.068 
Newman  Football  115  615  0.084 
Arenas  Jazz  198  2742  0.141 
Newman  Celegans N.  297  2359  0.054 
Arenas  Celegans M.  500  2025  0.016 
Arenas   1133  5451  0.009 
Newman  Polblogs  1224  19,087  0.026 
Newman  Netscience  1461  2742  0.003 
HetRec  Delicious  1861  7664  0.004 
Reverbnation & Twitter  Artists  2061  16916  0.008 
Newman  Power  4941  6594  0.001 
MovieLens & Rotten Tomatoes  IMDBratings  5350  2,027,990  0.142 
Stanford  Wikivote  7115  100762  0.004 
IMDB  IMDBactors  133,365  2,296,224  2.63e\(\)04 
OQC  DBLP  300,647  807,700  1.79e\(\)05 
5.1 Evaluation of the search methods
The main goals of this subsection are to evaluate (1) the hillclimber’s ability to find a pattern with (near)maximal interestingness, and (2) scalability of the algorithms, with a focus on the exact A*like algorithm.
Since the hillclimber depends on an initial seeding step, our first experiment investigates the effectiveness of the three seeding strategies. Table 2 shows the interestingness of the top1 pattern and the time needed to compute it when using the hillclimber with any of the three seeding strategies, using \(k = 1\), 10, or 100 seeds for ‘Uniform’ and ‘TopK’ seeding. The numbers shown are averages over results obtained on all but the two largest datasets (IMDBactors and DBLP), which are too large to run with ‘All’ seeding within a reasonable time.
Comparison of different seeding strategies for the hillclimber
k  SSGc  SSGi  

TopK  Uniform  TopK  Uniform  
1  11.58 (3.3 s)  10.10 (2.2 s)  5.30 (22.4 s)  1.45 (0.5 s) 
10  11.64 (15.5 s)  11.27 (21.3 s)  5.45 (27.9 s)  4.77 (4.8 s) 
100  11.67 (139.9 s)  11.62 (188.4 s)  5.45 (156.9 s)  5.43 (29.4 s) 
All  11.67 (11,120 s)  5.45 (2035 s) 
Comparison of search methods for finding the top1 subgraph, using SSGc
Search  #Cands  Time (s)  Int. 

Exhaustive (\(W \le 6\))  836,010,454  216.3  0.82 
Exact (A*)  3,497,690  8.2  1.05 
Hillclimber (\(k=10\))  1304  \({<}\)1  1.04 
Hillclimber (\(k=100\))  9130  \({<}\)1  1.05 
Scalability of the exact A* algorithm on random Erdős–Rényi graphs, parametrised by the number of vertices n and edge probability p
n  \(p = 0.1\)  \(p = 0.01\)  \(p = 0.001\)  

#Cands  t (s)  #X  #Cands  t (s)  #X  #Cands  t (s)  #X  
10  30  0  –  1  0  –  0  0  – 
20  207  0  –  13  0  –  1  0  – 
30  1501  0  –  27  0  –  1  0  – 
40  8939  0  –  538  0  –  2  0  – 
50  45,700  0  –  1412  0  –  15  0  – 
60  385,620  0  –  799  0  –  26  0   
70  2,009,217  2.5  –  3552  0  –  32  0  – 
80  6,931,587  12.6  –  7181  0  –  38  0  – 
90  22,126,867  46.9  –  11,035  0  –  40  0  – 
100  113,402,482  373.1  2  106,637  0  –  941  0  – 
125  –  –  10  1,302,221  1.2  –  10,618  0  – 
150  –  –  10  59,018,496  87  –  24,682  0  – 
175  –  –  10  830,066,282  1699  –  1,337,039  1.3  – 
200  –  –  10  615,623,653  1226  4  939  0  – 
225  –  –  10  97,129,173  235  9  457  0  – 
250  –  –  10  –  –  10  2102  0  – 
300  –  –  10  –  –  10  7006  0  – 
350  –  –  10  –  –  10  43,754  0  – 
400  –  –  10  –  –  10  362,809  0.7  – 
450  –  –  10  –  –  10  49,612,662  144  – 
500  –  –  10  –  –  10  107,787,526  325  – 
550  –  –  10  –  –  10  310,399,214  1067  4 
600  –  –  10  –  –  10  –  –  10 
Although exact search with the A*based algorithm is only feasible on moderately sized graphs, i.e. containing up to 100 s of vertices, the results in Table 3 show that pruning the search space is essential in making this possible. Compared to exhaustive enumeration of subgraphs containing up to six vertices, both the number of candidates and the computation time is reduced by two orders of magnitude. In other words, for moderately sized graphs the exact algorithm, including its pruning strategy, is an essential contribution as it enables the discovery of optimal patterns. We have to resort to heuristics to be able to discover patterns in larger networks though.
5.2 Evaluation of the interestingness measure
For the remaining experiments we will use the hillclimber with TopK seeding (\(k=10\)), as the previous subsection showed this search strategy to be very fast while closely approximating the optimal result. Moreover, it can also be used hasslefree on larger networks.
5.2.1 Effect of the prior beliefs
Here we investigate the effect of incorporating different kinds of prior beliefs by comparing SSGc and SSGi on all datasets considered (see Tables 5, 7). From Table 5 we observe that the average degree of the vertices in the most interesting subgraph according to SSGc is almost always higher than when using SSGi. This is to be expected, since for SSGi high degrees may represent a partial explanation for high density and thus reduce the information content. But also intuitively this makes perfect sense: different prior beliefs about the data should lead to different results, and our subjective interestingness measure allows for this. Second, we observe that interestingness under SSGc is typically higher than under SSGi. This should be no surprise either, given the fact that the user knows less about the data and hence has more to learn about it. This explanation is corroborated by the fact that the difference in interestingness is larger if the difference in average degree is larger as well.
Comparison of the most interesting patterns identified using different prior beliefs, SSGc and SSGi
Dataset  Method  Time (s)  Int.  AvgDeg. 

Karate  SSGi  \({<}\)1  0.61  3.3 
SSGc  \({<}\)1  0.55  9.0  
Dolphins  SSGi  \({<}\)1  0.67  3.7 
SSGc  \({<}\)1  0.76  8.2  
Lesmis  SSGi  \({<}\)1  1.50  8.3 
SSGc  \({<}\)1  1.69  13.9  
Polbooks  SSGi  \({<}\)1  1.28  6.6 
SSGc  \({<}\)1  0.98  18.1  
Adjnoun  SSGi  \({<}\)1  0.61  7.0 
SSGc  \({<}\)1  0.85  24.7  
Football  SSGi  \({<}\)1  1.99  10.8 
SSGc  \({<}\)1  1.42  11.3  
Jazz  SSGi  \({<}\)1  3.13  42.1 
SSGc  \({<}\)1  3.95  46.1  
Celeg. N  SSGi  \({<}\)1  1.64  15.0 
SSGc  \({<}\)1  1.88  44.4  
Celeg. M  SSGi  \({<}\)1  1.75  4.0 
SSGc  \({<}\)1  3.39  58.1  
 SSGi  \({<}\)1  3.28  20.2 
SSGc  \({<}\)1  4.04  20.2  
Polblogs  SSGi  1  2.60  94.2 
SSGc  1  11.59  107.9  
Netscience  SSGi  \({<}\)1  4.86  19.2 
SSGc  \({<}\)1  9.40  19.2  
Delicious  SSGi  \({<}\)1  5.97  18.4 
SSGc  \({<}\)1  9.27  39.6  
Artists  SSGi  1  7.60  56.4 
SSGc  \({<}\)1  18.53  123.8  
Power  SSGi  \({<}\)1  1.37  6.4 
SSGc  \({<}\)1  1.74  7.6  
IMDBratings  SSGi  438  49.46  632.0 
SSGc  231  105.00  1815.8  
Wikivote  SSGi  35  4.35  57.9 
SSGc  32  22.80  219.8  
IMDBactors  SSGi  479  22.05  134.3 
SSGc  481  14.67  143.1  
DBLP  SSGi  118  4.69  79.8 
SSGc  34  3.38  76.4 
5.2.2 Iterative pattern mining
As explained in Sect. 2.3.1, our approach is naturally suited for iterative application, as patterns presented in previous iterations can be incorporated into the background distribution for subsequent iterations. Table 6 shows some characteristics of the first 10 patterns found in this way, using SSGi (i.e. initially incorporating prior beliefs on the individual vertex degrees). Besides total computation time, the table shows the proportion of the graph covered by the union of the 10 subgraphs (‘coverage’), and the average Jaccard index over all pairs of subgraphs. The average Jaccard shows that while overlap tends to be avoided, small overlaps do take place. This illustrates how incorporating the presented patterns into the background distribution helps to avoid redundancy in the resulting pattern set, while patterns can still overlap when this is informative. Coverage varies strongly depending on the dataset, suggesting that our measure adapts itself to the scale and the structure of the dataset. The smaller datasets could be completely ‘explained’ with tens of patterns, whereas more patterns would be required to cover the larger graphs.
Characteristics of the first 10 patterns found by iterative mining using SSGi
Dataset  Time (s)  Coverage (%)  AvgJaccard 

Karate  \({<}\)1  50.0  0.959 
Dolphins  \({<}\)1  48.4  0.980 
Lesmis  \({<}\)1  59.7  0.995 
Polbooks  \({<}\)1  50.5  0.998 
Adjnoun  \({<}\)1  24.1  0.996 
Football  \({<}\)1  70.4  1.000 
Jazz  \({<}\)1  63.6  0.987 
Celegans N.  \({<}\)1  21.2  0.998 
Celegans M.  \({<}\)1  10.0  0.993 
 1  10.2  1.000 
Polblogs  5  16.4  0.995 
Netscience  \({<}\)1  7.0  1.000 
Delicious  \({<}\)1  8.1  1.000 
Artists  3  12.0  0.999 
Power  \({<}\)1  2.6  0.999 
IMDBratings  6080  35.4  0.975 
Wikivote  249  15.9  0.996 
IMDBactors  3428  1.7  0.999 
DBLP  879  0.4  1.000 
5.2.3 Comparison with alternative approaches
Properties of the most interesting pattern according to SSGc, SSGi, OQCG, and OQCL, i.e. interestingness as computed by SSGi and SSGc, size, edge density, diameter, and triangle density of each best subgraph discovered
Dataset  SSGi int.  SSGc int.  Size (W)  Edge density  Diameter  Triangle density  

Method  SSGi  OQCG  SSGc  OQCG  SSGi  SSGc  OQCG  OQCL  SSGi  SSGc  OQCG  OQCL  SSGi  SSGc  OQCG  OQCL  SSGi  SSGc  OQCG  OQCL 
Karate  0.61  0.01  0.55  0.16  3  5  10  6  1  1  0.56  0.93  1  1  3  2  1  1  0.18  0.80 
Dolphins  0.67  0.19  0.76  0.32  3  6  13  9  1  0.93  0.47  0.64  1  2  3  3  1  0.80  0.12  0.26 
Lesmis  1.50  0.13  1.69  0.65  8  9  22  13  1  1  0.51  0.88  1  1  2  2  1  1  0.19  0.72 
Polbooks  1.28  0.52  0.98  0.71  5  7  16  15  1  0.95  0.58  0.61  1  2  2  2  1  0.86  0.20  0.23 
Adjnoun  0.61  0.04  0.85  0.49  3  6  16  12  1  0.93  0.48  0.56  1  2  3  2  1  0.80  0.11  0.17 
Football  1.99  1.90  1.42  1.42  9  9  9  11  1  1  1  0.80  1  1  1  2  1  1  1  0.47 
Jazz  3.13  0.40  3.95  0.91  27  30  57  48  1  1  0.55  0.64  1  1  2  2  1  1  0.24  0.35 
Celeg. N.  1.64  0.46  1.88  1.78  8  14  22  24  0.89  0.80  0.61  0.58  2  2  2  2  0.70  0.52  0.26  0.22 
Celeg. M.  1.75  0.05  3.39  3.32  5  22  27  26  0.90  0.64  0.55  0.57  2  2  2  2  0.70  0.29  0.21  0.21 
 3.28  3.28  4.04  4.04  12  12  12  5  1  1  1  0.70  1  1  1  2  1  1  1  0.30 
Polblogs  2.60  2.05  11.59  10.96  55  70  100  98  0.72  0.71  0.55  0.56  2  2  2  2  0.40  0.38  0.20  0.21 
Netscience  4.86  4.86  9.40  9.39  20  20  20  8  1  1  1  0.71  1  1  1  2  1  1  1  0.43 
Delicious  5.97  3.27  9.27  9.19  19  44  40  24  0.99  0.60  0.64  0.51  2  2  2  3  0.97  0.25  0.30  0.17 
Artists  7.60  1.88  18.53  17.49  36  65  76  2  0.99  0.81  0.70  1  2  2  2  1  0.96  0.58  0.41  0.00 
Power  1.37  1.46  1.74  2.00  15  11  13  4  0.38  0.56  0.51  0.83  2  2  3  2  0.09  0.23  0.16  0.50 
IMDBratings  49.46  0.35  105.00  43.58  187  778  1937  1907  1  1  0.59  0.60  1  1  2  2  1  1  0.28  0.29 
Wikivote  4.35  1.23  22.80  20.82  141  240  133  117  0.19  0.32  0.48  0.50  2  2  2  2  0.01  0.05  0.13  0.15 
IMDBactors  22.05  16.56  14.67  12.38  291  259  192  29  0.37  0.43  0.52  0.50  2  2  3  3  0.01  0.01  0.02  0.04 
DBLP  4.69  4.64  3.38  3.38  98  75  75  7  0.72  1  1  1  2  1  1  1  0.47  1  1  1 
Average  6.28  2.28  11.36  7.53  49.7  88.5  146.8  124.5  0.85  0.83  0.65  0.69  1.5  1.6  2.1  2.1  0.75  0.67  0.37  0.34 
The results are summarised in Table 7. The leftmost columns contain SSGi resp. SSGc interestingness values as computed on the best patterns found by the SSGi resp. SSGc hillclimber and OQCG. For OQC, we restrict our focus to the G variant because it is deterministic and hence always produces the same results. The purpose of this comparison is twofold: (1) to show that our interestingness formalisation is different from that of OQC, and (2) to show that our hillclimber finds better patterns according to our interestingness criteria. Both claims are clearly confirmed by the results, as we explain next.
OQC is conceptually closer to SSGc than to SSGi and on some datasets, such as Football, Email, and DBLP, it finds results that are equally good to those found by the SSGc hillclimber. On average, however, our SSGc hillclimber scores much better than OQCG: 11.36 versus 7.53. With its more detailed prior belief, SSGi aims at another range of patterns and succeeds in finding patterns that score much higher than OQCG. On IMDBratings, for example, the pattern found by OQCG gets a score of only 0.35, whereas our SSGi hillclimber finds a subgraph with score 49.46. This demonstrates the power of our subjective interestingness measure, which can in principle be used in combination with a variety of prior beliefs, each of which results in different patterns.
Next, we compare the sizes of the best patterns found by the different algorithms. The size of the most interesting patterns according to SSG is sometimes smaller and sometimes larger when compared to OQC. SSG does tend to find subgraphs with higher edge densities though (with a few exceptions). The diameters for SSG are occasionally smaller but generally comparable. Most importantly, the triangle densities tend to be considerably higher for both SSG methods than for the OQC methods (again, with a few exceptions).
To sum up, in Tsourakakis et al. (2013) it was shown that OQC finds subgraphs that are denser than maximum density subgraphs (Goldberg 1984), but we here demonstrate that SSG often finds even denser subgraphs. And most importantly, SSG can use different prior belief sets, which leads to different results, as also indicated by the SSG interestingness results. Concretely, when SSGi is used the resulting dense subgraphs generally do not contain any of the highestdegree vertices (hubs), because it is already known that they are located in dense regions of the graph (see also the average degrees in Table 5).
5.2.4 External evaluation
Here we investigate to which extent the patterns found in the IMDBratings resp. Artists datasets correspond to movie resp. music genres. Genre information was not used to generate the networks and this investigation can therefore be regarded as an external, independent evaluation. Of course, there is no guarantee that the most interesting patterns relate to movie or music genres as defined by humans. It is possible that movie tastes relate to movie properties other than genres as defined in the IMDB dataset, such as the actors playing, the director, or perhaps something less obvious. Similarly, there could be different reasons why music bands receive attention on Twitter than just the genre of their music. Thus, although the presence of an association between the patterns found and genres would be a validation of our approach, the absence of such an association could not be interpreted for the failure of the method.
Significant genres, negatively or positively associated, for the top10 patterns on IMDBratings using SSGi
Positively associated genres  Negatively associated genres  

1  Drama (9.1e\(\)09), Romance (3.7e\(\)10)  Horror (1.1e\(\)06) 
2  –  – 
3  SciFi (2.2e\(\)08), Thriller (2.1e\(\)11)  – 
4  FilmNoir (7.5e\(\)06)  Adventure (2.2e\(\)05), Horror (8.0e\(\)06) 
5  Drama (1.4e\(\)11)  Action (2.8e\(\)07), Horror (2.0e\(\)05), Thriller (2.3e\(\)05) 
6  Horror (6.1e\(\)09)  Romance (5.7e\(\)05) 
7  Drama (0), War (1.1e\(\)10)  Comedy (8.8e\(\)12) 
8  Crime (5.9e\(\)06)  Romance (8.8e\(\)06) 
9  Drama (9.4e\(\)13), Romance (0), War (7.4e\(\)07)  Children (5.8e\(\)05), Horror (3.6e\(\)13), Thriller (7.7e\(\)05) 
10  Musical (6.2e\(\)07), Romance (4.2e\(\)08)  Drama (8.7e\(\)05) 
Significant genres, negatively or positively associated, for the top10 patterns on IMDBratings using SSGc
Positively associated genres  Negatively associated genres  

1  Adventure (1.3e\(\)09), SciFi (1.0e\(\)05)  – 
2  Action (3.3e\(\)06), Adventure (4.7e\(\)05), SciFi (1.2e\(\)05), Thriller (1.5e\(\)12)  – 
3  Drama (1.8e\(\)08), FilmNoir (3.5e\(\)05), Romance (3.4e\(\)05)  Action (6.7e\(\)06), Horror (8.2e\(\)07) 
4  Drama (2.2e\(\)10), Romance (5.3e\(\)15), War (6.6e\(\)06)  Horror (8.6e\(\)08), Thriller (2.3e\(\)05) 
5  Drama (2.3e\(\)12), War (6.1e\(\)07)  Comedy (2.7e\(\)07) 
6  Drama (4.6e\(\)07)  Action (5.7e\(\)07), Horror (3.4e\(\)06) 
7  –  – 
8  Action (0), Adventure (2.4e\(\)10), Animation (3.7e\(\)13), Fantasy (4.6e\(\)06)  – 
9  Horror (1.6e\(\)07)  – 
10  Musical (3.1e\(\)08), Romance (6.9e\(\)08)  – 
Significant genres, negatively or positively associated, for the top10 patterns on IMDBratings using OQCG. Bonferroni corrected p values \({<}\)1e\(\)4 shown between brackets
Positively associated genres  Negatively associated genres  

1  Action (3.4e\(\)05), Adventure (6.6e\(\)13), Animation (1.5e\(\)05), Crime (6.0e\(\)05), Drama (1.4e\(\)08), Fantasy (9.1e\(\)08), Mystery (5.9e\(\)06), SciFi (1.3e\(\)05), Thriller (5.2e\(\)11)  – 
2  Drama (3.1e\(\)06), Romance (4.5e\(\)10)  Horror (2.3e\(\)05) 
3  Drama (1.2e\(\)05)  Action (1.1e\(\)07), Horror (9.9e\(\)09), SciFi (3.4e\(\)05), Thriller (6.6e\(\)05) 
4  –  Romance (2.7e\(\)05) 
5  Horror (1.1e\(\)06)  – 
6  Action (4.4e\(\)07), Animation (2.0e\(\)06), Musical (5.5e\(\)05)  – 
7  –  – 
8  Action (1.0e\(\)11), Fantasy (3.0e\(\)06), Horror (3.5e\(\)08), SciFi (4.7e\(\)09)  Drama (2.8e\(\)09) 
9  –  – 
10  –  – 
Significant genres, negatively or positively associated, for the top10 patterns on Artists using SSGi
Positively associated genres  Negatively associated genres  

1  Rock (0.0e + 00)  – 
2  Electronica (1.5e\(\)05), trance (0.0e + 00)  – 
3  Indie (3.5e\(\)07)  – 
4  Bhangra (2.0e\(\)14), world (8.8e\(\)03)  – 
5  Christian (3.0e\(\)14), christian rap (8.7e\(\)06), gospel (0.0e + 00)  – 
6  Country (0.0e + 00)  – 
7  Grime (8.6e\(\)09), hip hop (1.1e\(\)12), rap (4.1e\(\)11)  Rock (1.4e\(\)03) 
8  Afro pop (8.8e\(\)05)  – 
9  UK garage (2.4e\(\)03)  – 
10  Hip hop (1.4e\(\)10)  – 
Looking at the patterns in detail, SSGi appears to find more niche genres whereas SSGc and OQCG tend to find sets of associated blockbusters seen (and liked) by many. For example, although SSGc and OQCG do not find the same top pattern, the three highest degree vertices in their respective top patterns are Pulp Fiction, The Matrix, and Fight Club. On the other hand the three highest degree vertices in the top pattern of SSGi are the relatively unknown Orlando, Twelve O’Clock High, and Pieces of April. This is not surprising as SSGc and OQCG do not take into account the degree distribution.
The “–”s in Tables 8, 9 and 10 mean that there are no genres significantly associated with the respective patterns. However this does not mean that the movies in the pattern are not related but just that the pattern cannot be explained based on significant associations with genres. Upon closer inspection of these patterns, we noticed that pattern 7 in Table 9 contains films with a male main character, whereas pattern 2 in Table 8 contains old films mainly from the 60s, 70s and 80s. The full lists of movies in the top10 patterns for SSGi and SSGc on this dataset can be found in the supplementary material^{12}.
Tables 11, 12 and 13 show which genres are significantly associated with the top10 patterns found in the Artists dataset, for SSGi, SSGc and OQCG respectively. The p value is again computed using the hypergeometric test. As Bonferroni correction for multiple testing the result is multiplied with 181, which is the number of music genres which appear at least 3 times in this dataset. The significance threshold used now is 0.01 as the dataset is smaller.
Significant genres, negatively or positively associated, for the top10 patterns on Artists using SSGc
Positively associated genres  Negatively associated genres  

1  Alternative (6.6e\(\)03), indie (2.5e\(\)05)  – 
2  Grime (1.1e\(\)07), hip hop (1.2e\(\)12), rap (2.6e\(\)11)  Rock (4.3e\(\)03) 
3  Rock (1.4e\(\)13)  – 
4  Pop (6.2e\(\)05)  – 
5  Indie (9.9e\(\)06)  – 
6  UK garage (2.4e\(\)03)  – 
7  Electronica (1.5e\(\)05), trance (0.0e + 00)  – 
8  Christian (1.7e\(\)11), christian rap (4.3e\(\)06), gospel (6.6e\(\)11)  – 
9  –  – 
10  Country (0.0e + 00)  – 
Significant genres, negatively or positively associated, for the top10 patterns on Artists using OQCG
Positively associated genres  Negatively associated genres  

1  Indie (8.0e\(\)05)  – 
2  Grime (2.5e\(\)09), hip hop (7.2e\(\)13), rap (7.5e\(\)12), uk (4.0e\(\)04)  Rock (2.9e\(\)03) 
3  Rock (4.4e\(\)13)  – 
4  Indie (2.6e\(\)06)  – 
5  –  – 
6  Christian (5.8e\(\)05), christian rap (2.8e\(\)03), gospel (1.4e\(\)04)  – 
7  Electronica (4.0e\(\)06), trance (1.2e\(\)11)  – 
8  UK garage (2.8e\(\)03)  – 
9  Country (0.0e + 00)  – 
10  Bhangra (2.0e\(\)12)  – 
5.3 Practical guidance
The experiments suggest the following three possible usage scenarios for our subjective subgraph mining framework.
The first scenario is the most obvious one, in which the user is actually able to express certain prior beliefs about the data. The particular cases considered in this paper are prior beliefs about the individual vertex degrees, or the overall edge density of the graph. This may be rather demanding in practice, but often still possible. For example the overall edge density is easy to specify and it is conceivable that the user genuinely has a prior belief about it. Note that this scenario allows for the prior beliefs to be incorrect, and if that is the case the most interesting patterns are likely to be patterns that provide evidence to rectify those incorrect prior beliefs.
In a second scenario, the user starts by a ‘shallow’ exploration of the data, prior to searching for the dense subgraph patterns. For example, they may compute the overall edge density (or estimate it by random sampling), or they may compute and scrutinise the individual vertex degrees. The result of this is that this information becomes part of their prior beliefs, after which the first scenario applies.
The third scenario is best explained by means of an example. In the Artists graph used in the experiments above, the user may not actually hold easily quantifiable beliefs about the degree of each Artist in the network. Yet, they may consider the degrees as irrelevant, i.e. they may want to see patterns that cannot easily be explained by individual degrees. The rationale could be that this information is easily verified by means of a simple lookup, such that for all practical purposes it can be consider prior information. In this scenario, it makes sense for the user to ask the system to find the most interesting patterns pretending that they are aware of the individual vertex degrees. In this case, the prior beliefs used should be based on the actual data (i.e. the actual vertex degrees), as in the second scenario.
6 Conclusions
Dense subgraph mining, as an exploratory data mining task, has long eluded the fact that the interestingness of a dense subgraph pattern is inevitably a subjective notion. While previous research has attempted to approach the problem by approximating interestingness in a number of ‘objective’ ways, in this paper we explicitly recognise its subjective nature and formalise interestingness by contrasting the dense subgraph patterns with a background distribution that formalises the user’s prior beliefs about the data. For concreteness, we focus on two important specific kinds of prior belief sets. Furthermore, we show how the resulting background distributions can be updated efficiently to account for the knowledge of patterns already found, thus allowing for an iterative data mining approach.
This subjective interestingness approach has considerable advantages, most notably the fact that it automatically adapts itself to the user. While we pay a price in terms of computation times as compared to important alternatives, we do present a performant exact, and a highly scalable and accurate heuristic algorithm for mining the most interesting patterns according to our measures.
For further work, we plan to explore increasing the number of prior belief types that can be dealt with along the lines of the discussion in Sect. 2.3.1. Another interesting line of further work is the generalisation of the dense subgraph pattern syntax to the multirelational setting, which would result in a generalisation of the pattern syntax from Spyropoulou et al. (2014).
More practically, we anticipate that the proposed approach may lead to innovative applications in social media analysis, bioinformatics, recommendation systems, and many more. To highlight one possible application: in bioinformatics it has long been of interest to identify sets of coexpressed genes. This task is complicated by the fact that certain genes are expressed more often than others (e.g. housekeeping genes), such that any coexpression with these genes is less meaningful and potentially spurious. The strategy presented in the current paper could provide an innovative and natural way of dealing with that, when applied to a graph over the set of genes in which edges are an indication of coexpression. More generally, using our approach for such exploratory data mining problems where confounding factors (such as individual vertex degrees) are present, forms an exciting avenue for further work.
Footnotes
 1.
Note that we are interested in finding just the best pattern(s), rather than in enumerating them all as is common in the frequent pattern mining literature. The reason is precisely our focus on formalising subjective interestingness: if this is done adequately, by definition only the most interesting ones should be of interest to the user.
 2.
This model can be adapted to deal with graphs with selfedges, quite simply by changing \(u<v\) into \(u\le v\) below the product symbol. Additionally, it can be adapted to directed graphs. In that case, it is natural to assume prior beliefs on the indegrees as well as the outdegrees of the vertices. This would result in a distribution of the form \(P(E)=\prod _{u,v}\frac{\exp ((\lambda _u+\mu _v)\cdot a_{u,v})}{1+\exp (\lambda _u+\mu _v)}\), where \(a_{u,v}=1\) indicates the presence of an arc from u to v in E, and the \(\lambda \) parameters affect the outdegree probabilities and the \(\mu \) parameters the indegree probabilities. We refer to De Bie (2011b), for details.
 3.
Note that this optimisation problem will also always be feasible in our setting, as the value of k is found as \(\phi (E)\) on the actual data E, and hence a point distribution would always satisfy the constraint.
 4.
Note that the fact that \(0\le X_k\le 1\) in the general theorem suggests that it can be used also in a possible extension of our work for weighted graphs.
 5.
The bound only holds for \(\hat{p}>p\), but of course we are only interested in this situation (subgraphs that are denser than expected). The bound is tighter if the different values for \(p_{u,v}\) are more similar to each other, and thus in particular in the case where the user only holds a belief about the overall density, so that \(p_{u,v}=p\) for some constant p and \(p_W=p\).
 6.
Strictly speaking a small extra description of length \(\log (W)\) would be need to be added to account for encoding \(k_W\). However, for N or W sufficiently large this would become negligible, so we ignore it here for simplicity.
 7.
To be precise, Tsourakakis et al. (2013) actually define the edge density more generally, as a general parametric form of a subgraph interestingness measure, before proposing the specific form we reproduce here. Note however that our proposal is not a special case of that more general definition.
 8.
This bound could be tightened further without much loss of efficiency using a piecewise linear upper bound.
 9.
Note that we do not explicitly limit the set of tests to those for patterns with a short description. Instead, we just use it to bias the choice of pattern towards simpler ones. This is similar in spirit to regularisation in machine learning, where any bias is good to effectively limit the hypothesis space in order to enhance generalisation.
 10.
Data sources are: Newman: http://wwwpersonal.umich.edu/~mejn/netdata/; Arenas: http://deim.urv.cat/~aarenas/data/welcome.htm; Stanford: http://snap.stanford.edu/data/; HetRec: http://ir.ii.uam.es/hetrec2011/; OQC: kindly provided to us by the authors of Tsourakakis et al. (2013); IMDBactors: coactor graph directly extracted from http://www.imdb.com, available upon request; IMDBratings: combines movie data from http://grouplens.org/datasets/movielens/ with ratings from http://www.rottentomatoes.com/, two movies in the network are connected if at least two users gave both movies the maximal rating (5 out of 5), available upon request; Reverbnation Artists: constructed by using Twitter handles of music artists from http://www.reverbnation.com and making a network of artists where two artists are connected iff more than 10 people have tweeted about both of them.
 11.
Binaries and source code of SSG Miner are available for download at http://patternsthatmatter.org/software.php#ssgminer.
 12.
Available from http://patternsthatmatter.org/software.php#ssgminer.
 13.
Available from http://patternsthatmatter.org/software.php#ssgminer.
 14.
The respective results on the IMDBratings dataset could not be visualised due to the larger dataset and patterns.
Notes
Acknowledgments
We gratefully acknowledge discussions with Jefrey Lijffijt which have helped us to improve the presentation of this manuscript. This work was funded by a Postdoctoral Fellowship of the Research Foundation Flanders (FWO), the European Research Council through the ERC Consolidator Grant FORSIED (Project Reference 615517), and by the EPSRC Project DS4DEMS (EP/M000060/1).
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