Centrality measures in networks

Abstract

We show that prominent centrality measures in network analysis are all based on additively separable and linear treatments of statistics that capture a node’s position in the network. This enables us to provide a taxonomy of centrality measures that distills them to varying on two dimensions: (i) which information they make use of about nodes’ positions, and (ii) how that information is weighted as a function of distance from the node in question. The three sorts of information about nodes’ positions that are usually used—which we refer to as “nodal statistics”—are the paths from a given node to other nodes, the walks from a given node to other nodes, and the geodesics between other nodes that include a given node. Using such statistics on nodes’ positions, we also characterize the types of trees such that centrality measures all agree, and we also discuss the properties that identify some path-based centrality measures.

Appendices

Appendix: proofs

Proof of Lemma 1

The if part is clear, and so we show the only if part.

Suppose that $s_i({\textbf{g}}) = s_i({\textbf{g}}^{\prime })$ but $c_i({\textbf{g}}) \ne c_i({\textbf{g}}^{\prime })$. Without loss of generality let $c_i({\textbf{g}})>c_i({\textbf{g}}^{\prime })$. By monotonicity, since $c_i({\textbf{g}})>c_i({\textbf{g}}^{\prime })$ it must be that $s_i({\textbf{g}}^{\prime }) \nsucceq s_i({\textbf{g}})$. However, this contradicts the fact that $s_i({\textbf{g}}) = s_i({\textbf{g}}^{\prime })$ (which implies that $s_i({\textbf{g}}) \sim s_i({\textbf{g}}^{\prime })$ by reflexivity of a partial order). Thus, $c_i({\textbf{g}}) = c_i({\textbf{g}}^{\prime })$ for any ${\textbf{g}},{\textbf{g}}^{\prime }$ for which $s_i({\textbf{g}}) = s_i({\textbf{g}}^{\prime })$. Letting S denote the range of $s_i({\textbf{g}})$ (which is the same for all i by anonymity), it follows that there exists ${\mathcal {C}}_i:S\rightarrow {\mathbb{R}}$ for which $c_i({\textbf{g}})={\mathcal {C}}_i(s_i({\textbf{g}}))$ for any ${\textbf{g}}$. Moreover ${\mathcal {C}}_i$ must be a monotone function on S, given the monotonicity of $c_i$.

Next, we show that ${\mathcal {C}}_i={\mathcal {C}}_j$ for any i, j. Consider any $s'\in S$ and any two nodes i and j. Since $s'\in S$, it follows that there exists ${\textbf{g}}$ for which $s_i({\textbf{g}})=s'$. Consider a permutation $\pi$ such that $\pi (j)=i$ and $\pi (i)=j$. Then by the anonymity of s, $s' =s_j({\textbf{g}}\circ \pi )$. Thus, by anonymity of c, $c_i({\textbf{g}}) = c_j({\textbf{g}}\circ \pi )$ and so ${\mathcal {C}}_i(s')={\mathcal {C}}_j(s')$. Given that $s'$ was arbitrary, it follows that ${\mathcal {C}}_i={\mathcal {C}}_j={\mathcal {C}}$ for some ${\mathcal {C}}:S\rightarrow {\mathbb{R}}$ and all i, j.

We extend the function ${\mathcal {C}}$ to be monotone on all of ${\mathbb{R}} ^L$ as follows. Let $S_1$ be the set of $s\notin S$ such that there exists some $s'\in S$ for which $s'\ge s$. For any $s\in S_1$ ${\mathcal {C}}_i(s)=\inf _{s'\in S, s'\ge s} {\mathcal {C}}(s')$. Next, let $S_2$ be the set of $s\notin S \cup S_1$. For any $s\in S_2$ let ${\mathcal {C}}(s)=\sup _{s'\in S \cup S_1, s'\le s} {\mathcal {C}}(s')$ (and note that this is well defined for all $s\in S_2$ since there is always some $s'\in S \cup S_1, s'\le s$). This is also monotone, by construction. $\square$

Proof of Theorems 1-3.

IF part:

It is easily checked that if ${\mathcal {C}}$ can be expressed as in equation (2), then independence holds. Similarly, if the representation is as in (3), then recursivity also holds, as does additivity if (4) is satisfied.

ONLY IF part:

Let $\textbf{e}^{\ell }$ denote the vector in ${\mathbb {N}}^{L}$ with every $\textbf{e}_{\ell }^{\ell }=1$ and $\textbf{e}_{j}^{\ell }=0$ for all $j\not =\ell$. Define $F_{\ell }:{\mathbb {R}}\rightarrow {\mathbb {R}}_{+}$ as

$$\begin{aligned} F_{\ell }\left( x\right) ={\mathcal {C}}\left( x\textbf{e}^{\ell }\right) . \end{aligned}$$

(7)

Iterated applications of independence imply that

$$\begin{aligned} {\mathcal {C}}\left( s_{i}\right) =\sum _{\ell =1}^{L} F_{\ell }\left( s_{i,\ell }\right) . \end{aligned}$$

(8)

To see this, note that for $s_{i}=\left( x,0,\ldots ,0,\ldots ,\right)$, $s_{i}^{\prime }=\left( 0,y,0,\ldots ,0,\ldots ,\right)$, and $s_{i}^{\prime \prime }=\left( x,y,0,\ldots ,0,\ldots ,\right)$, independence requires

$$\begin{aligned}{} & {} {\mathcal {C}}\left( s_{i}^{\prime \prime }\right) -F_{2} \left( y\right) =F_{1}\left( x\right) -0, \\{} & {} {\mathcal {C}}\left( s_{i}^{\prime \prime }\right) =F_{1} \left( x\right) +F_{2}\left( y\right) . \end{aligned}$$

Doing this again for $s_{i}=\left( x,y,0,0,\ldots ,0,\ldots ,\right)$, $s_{i}^{\prime }=\left( x,0,z,0,\ldots ,0,\ldots ,\right)$, and $s_{i}^{\prime \prime }=\left( x,y,z,0,\ldots ,0,\ldots ,\right)$, independence requires

$$\begin{aligned} {\mathcal {C}}\left( s_{i}^{\prime \prime }\right) =F_{1} \left( x\right) +F_{2}\left( y\right) +F_3(z). \end{aligned}$$

By induction, this holds for arbitrary vectors. Monotonicity implies that $F_{\ell }$ is increasing and $F_{\ell }\left( 0\right) =0$ for all $\ell$.

By recursivity, for all $\ell \le L$ and all x in ${\mathbb {R}}$:

$$\begin{aligned} \frac{F_{\ell +1}\left( x\right) }{F_{\ell }\left( x\right) } =\frac{F_{2}\left( x\right) }{F_{1}\left( x\right) } =\delta \left( x\right) . \end{aligned}$$

(9)

Moreover, recursivity also implies that for any two $x,x^{\prime }$ in ${\mathbb{R}}$ and any $\ell$ (provided the denominators are not 0):

$$\begin{aligned} \frac{F_{\ell }\left( x^{\prime }\right) }{F_{\ell }\left( x\right) } =\frac{F_{1}\left( x^{\prime }\right) }{F_{1}\left( x\right) }. \end{aligned}$$

(10)

From (9) this is true only if $\delta \left( x\right) \equiv \delta$ is constant.

Next, note that (10), together with the fact $F_\ell (0)=0$ for all $\ell$, imply that $F_\ell (x)=\delta ^\ell f(x)$ for a common f for which $f(0)=0$. This implies that ${\mathcal {C}}$ can be written as in equation (3).

Finally, additivity—which clearly implies independence—then implies that f is linear (a standard result in vector spaces), and given that it must be that $f(0)=0$, the final characterization follows. $\square$

Proof of Theorems 4and 5. The “if” parts of both theorems are straightforward, and so we prove the “only if” claims, beginning with Theorem 4.

From Theorem 3 it follows that

$$\begin{aligned} c_{i}\left( g\right) ={\mathcal {C}}(s_{i}\left( g\right) ) =a \sum _{\ell =1}^{L}\delta ^{\ell -1}s_{i}^{\ell }\left( g\right) . \end{aligned}$$

The strictness of sensitivity implies that $\delta >0$ and $a>0$. It suffices to show that $s_i({\textbf{g}})$ must be a weighted neighborhood statistic that is $\delta$-monotone.

Cycle independence implies that for any ${\textbf{g}}$, $c_i({\textbf{g}}) = c_i({\textbf{g}}')$ where ${\textbf{g}}'$ is any minimal i-centered subtree of ${\textbf{g}}$. Thus, it suffices to prove the characterization for trees.

Let us construct a series of line networks, with i at one extreme and with k links, denoted $g^k$. Iteratively, for $k=1$ to $n-1$ define

$$\begin{aligned} w_i^k \equiv \frac{\sum _{\ell =1}^{L}\delta ^{\ell -1}s_{i}^{\ell } \left( g^k\right) - \sum _{\ell =1}^{L}\delta ^{\ell -1}s_{i}^{\ell } \left( g^{k-1}\right) }{\delta ^\ell }. \end{aligned}$$

It follows directly (noting that $n_{i}^{\ell }\left( g^k\right) =1$ for $\ell \le k$ and 0 otherwise) that

$$\begin{aligned} c_{i}\left( g^k\right) =a \sum _{\ell =1}^{L} \delta ^{\ell -1} w_i^\ell n_{i}^{\ell }\left( g^k\right) . \end{aligned}$$

Any i-centered tree ${\textbf{g}}$ with depth (maximum distance to i) of k, can be built from $g^k$ by a successive addition of links. Iteratively building ${\textbf{g}}$ from $g^k$ by adding links that connect to the tree present at each step, and applying the constant marginal values condition at each step, then implies that

$$\begin{aligned} c_{i}\left( g\right) =a \sum _{\ell =1}^{L}\delta ^{\ell -1} w_i^\ell n_{i}^{\ell }\left( g\right) . \end{aligned}$$

Finally, comparing $c_i(g^k)$ to $c_i(g^{k-1}+ hj)$ where h is at distance $k-2$ and j is not in the network $g^{k-1}$, implies that $\delta ^{\ell -1} w_i^\ell > \delta ^{\ell } w_i^{\ell +1}$.

The proof of Theorem 5 comes from changing the final step above, which implies instead that $\delta ^{\ell -1} w_i^\ell =0$ for all $\ell >1$. $\square$

Proof of Proposition 1

[IF] We first prove the ‘if’ part. Consider a monotone hierarchy and two nodes i, j.

First, suppose that $i \doteq j$. From the definition of $\doteq$ it must be that i and j are at the same level of the hierarchy, and that any subgraph containing i starting at the same level as some subgraph containing j are identical (up to the labeling of the nodes). Hence i and j are symmetric, and $n_i({\textbf{g}}) = n_j({\textbf{g}})$.

So, to complete the proof of the ‘if’ part, it is sufficient to show that if $i \gtrdot j$ then $n_i \succ n_j$.

Consider two nodes at the same distance from the root.

If they are at distance 1, then they have the same distance to each node that is not a successor of either node. Given the definition of monotone hierarchy, it must be that either $d_i>d_j$ or that $d_i=d_j$ and $d_k > d_l$ for some successor k of i and any successor l of j such that $\rho (k)= \rho (l)$. In either case the definition implies that then $d_k \ge d_l$ for every successor k of i and successor l of j such that $\rho (k)= \rho (l)$. It directly follows that $n_i \succ n_j$.

Inductively, if $\rho (i) = \rho (j)>1$:

• If there exist two distinct predecessors k, l of i, j, respectively, such that $\rho (k) = \rho (l)$ and $k \gtrdot l$, then $i \gtrdot j$, then the ordering holds given the ordering of those predecessors and that their neighborhoods are determined by those predecessors.

• Otherwise, they follow from a common immediate predecessor and differ only in the subgraphs starting from them, and the condition follows from the reasoning above given the differences in those subgraphs, which must be ordered.

Next, suppose that $\rho (j) = \rho (i)+1$. We show that $n_i({\textbf{g}}) \succ n_j({\textbf{g}})$.

For this part of the proof we provide a formula to compute the number of nodes at distance less than or equal to d from node i for a monotone hierarchy, Q(i, d). Let $\rho (i)$ denote the distance from the root and $i_0,i_1,..,i_k,\ldots ,i_{\rho (i)}=i$ the unique path between the root and node i. Let $p(i,\ell )$ denote the number of successors of node i at distance $\ell$. If $d \ge \rho (i)$, we compute the number of nodes at distance less than or equal to d as

$$\begin{aligned} Q(i,d)= & {} p(i_0,0) + p(i_0,1)+\cdots + p(i_0, d-\rho (i)) \\+ & {} p(i_1,d-\rho (i)) + p(i_1, d-\rho (i)+1)+p(i_2, d -\rho (i)+1) + p(i_2, d-\rho (i)+2) \\+ & {} \ldots p(i_{\rho (i)-1}, d-2) + p(i_{\rho (i)-1}, d-1) + p(i,d-1)+ p(i,d). \end{aligned}$$

To understand this computation, notice that all nodes which are at distance less than or equal to $d-\rho (i)$ from the root are at a distance less than d from node i. Other nodes at a distance less than d from node i are computed considering the path between $i_0$ and i. Fix $i_1$. There are successor nodes which are at distance $d-\rho (i)$ from node $i_1$ (and hence at a distance $d-1$ from i) and were not counted earlier because they are at a distance of $d-\rho (i)+1$ from the root, and successor nodes which are at a distance $d-\rho (i)+1$ from node 1 (and hence at a distance d from i) and were not counted earlier because they are at a distance $d- \rho (i)+2$ from the root. Continuing along the path, for any node $i_k$ we count successor nodes at a distance $d-\rho (i)+k-1$ and $d-\rho (i)+k$ from node $i_k$ which are at a distance $d-1$ and d from node i and were not counted earlier, and finally obtain the total number of nodes at a distance less or equal to d from node i.

Next suppose that $d \le \rho (i)$. In that case, no node beyond $i_0$ who does not belong to the subtree starting at $i_1$ can be at a distance smaller than d. The expression for the number of nodes at a distance less than or equal to d simplifies to

$$\begin{aligned} Q(i,d)= & {} p(i_{\rho (i)-d},0) + p(i_{\rho (i)-d+1},0) + p(i_{\rho (i)-d+1}, 1) \\+ & {} \ldots p(i_{\rho (i)-1},d-2)+ p(i_{\rho (i)-1},d-1) +p(i,d-1) + p(i,d). \end{aligned}$$

The following claim is useful.

Claim 1

In a monotone hierarchy, for any i, j such that $\rho (j)=\rho (i)+1$, and any $\ell$, $p(i,\ell ) \ge p(j,\ell )$.

Proof of the Claim

The proof is by induction on $\ell$. For $\ell =1$, the statement is true as $p(i,1) \equiv d_i-1 \ge d_j-1 \equiv p(j,1)$. Suppose that the statement is true for all $\ell ^{\prime }<\ell$. Let $i_1,..,i_I$ be the direct successors of i and $j_1,..,j_J$ the direct successors of j, with $J \le I$. Then

$$\begin{aligned} p(i, \ell )= & {} \sum _{r=1}^I p(i_r,\ell -1), \\\ge & {} \sum _{r=1}^J p(i_r,\ell -1), \\\ge & {} \sum _{r=1}^J p(j_r,\ell -1) \\= & {} p(j, \ell ). \end{aligned}$$

where the first inequality is due to the fact that $I \ge J$ and the second that, by the induction hypothesis, as $\rho (i_r) = \rho (j_r)-1$ for all r, $p(i_r,\ell ^{\prime }-1) \ge p(j_r, \ell ^{\prime }-1)$. $\square$

Consider $d \ge \rho (i)+1$ and $i_0,i_1,.i_r,.,i_{\rho (i)}$, $i_0,j_1,\ldots ,j_r, j_{\rho (i)+1}$ the paths linking i and j to the root. Then

$$\begin{aligned}{} & {} Q(i,d)-Q(j,d)\\{} & {} \quad = p(i_0, d- \rho (i))-p(j_1, d-\rho (i)-1) - p(j_1, d- \rho (i)) \\{} & {} \qquad + [p(i_1,d-\rho (i))+ p(i_1, d-\rho (i)+1)-p(j_2,d-\rho (i)) - p(j_2, d- \rho (i)+1) ]\\{} & {} \qquad +\ldots [p(i_r, d-\rho (i)+r-1) + p(i_r, d-\rho (i)+r) \\{} & {} \qquad - p(j_{r+1}, d- \rho (i)+r-1)- p(j_{r+1}, d- \rho (i)+)] \\{} & {} \qquad +\ldots [p(i,d-1) +p(i,d)-p(j,d-1) -p(j,d)] \end{aligned}$$

Note that $p(i_0, d-\rho (i))= p(j_1, d-\rho (i)-1) + \sum _{k \ne j_1, \rho (k)=1} p(k, d- \rho (i)-1)$ and that $p(j_1, d- \rho (i)) =\sum _{l| \rho (l)=2, \rho (j_1,l)=1} p(l, d- \rho (i)-1)$. By Claim 1, as $\rho (l)= \rho (k+1)$, $p(l, d- \rho (i)-1) \le p(k, d- \rho (i)-1)$ and as $d(j_1) \le d(i_0)-1$, $\sum _{k \ne j_1, \rho (k)=1} p(k, d- \rho (i)-1) \ge \sum _{l| \rho (l)=2, \rho (j_1,l)=1} p(l, d- \rho (i)-1)$. Furthermore, by Claim 1, for all r and all d, $p(i_r,d) \ge p(j_{r+1},d)$, so that $Q(i,d)-Q(j,d) \ge 0$.

Next, consider $d \le \rho (i) < \rho (i)+1$. Then

$$\begin{aligned} Q(i,d)-Q(j,d)= & {} [p(i_{\rho (i)-d},0) + p(i_{\rho (i)-d+1},0) -p(j_{\rho (i)-d+1},0) - p(j_{\rho (i)-d+2},0)] \\{} & {} +\ldots [p(i_{\rho (i)-1},d-1)+p(i,d-1) - p(j_{\rho (i)},d-1) - p(j,d-1)] \\{} & {} + [p(i,d)-p(j,d)] \end{aligned}$$

and by a direct application of Claim 1, $Q(i,d)-Q(j,d) \ge 0$.

We finally observe that there always exists a distance d such that $Q(i,d)>Q(j,d)$. Let h be the total number of levels in the hierarchy. Consider a distance d such that $h=d + \rho (i)$. Then there exist successor nodes at distance d from i but no successor nodes at distance d from j. Hence $p(i,d)>0 = p(j,d)$. This establishes that $Q(i,d) > Q(j,d)$ and hence $n_i({\textbf{g}}) \succ n_j({\textbf{g}})$. By a repeated application of the same argument, for any i, j such that $\rho (i) < \rho (j)$, for any i, j such that $\rho (i,i_0) < \rho (j,i_0)$, $n_i({\textbf{g}}) \succ n_j({\textbf{g}})$.

[ONLY IF]: Suppose that the tree ${\textbf{g}}$ is not a monotone hierarchy and has an even diameter. Consider a line in the tree which has the same length as the diameter of the tree. Pick as a root the unique middle node in the line and let h be the maximal distance between the root and a terminal node.

First consider the case in which there exist two nodes i and j such that $\rho (j)= \rho (i)+1$ but $d_j > d_i$. Then clearly $Q(j,1) > Q(k,1)$. Notice that all nodes are at a distance less than or equal to $d=h+\rho (i)$ from node i whereas there exist nodes which are at a distance $h+ \rho (i)+1$ from node j, and hence $Q(j,h+\rho (i)) < Q(i,h+ \rho (i))$ so that neither $n_i \succeq n_j$ nor $n_j \succeq n_i$.

Next suppose that for all nodes i, j such that $\rho (j)=\rho (i)+1$, $d_j \le d_i$, but that there exists two nodes i, j at the same level of the hierarchy such that $d_i > d_j$ and two successors of i and j, k and l, at the same level of the hierarchy such that $d_k < d_l$. Because $d_i > d_j$, $Q(i,1)>Q(j,1)$. Suppose that $n_i \succ n_j$. Then $Q(i,d) > Q(j,d)$ fr all $d=1,2,\ldots ,h+ \rho (i)-1$. Now consider the two successors k and l of i and j. As $d_k < d_l$, $Q(k,1) < Q(l,1)$. Now count all the nodes which are at a distance less than or equal to $h+\rho (k)-1$ from k, $Q(k,h+\rho (k)-1)$. This includes all the nodes but the nodes which are at maximal distance from k. As k is a successor of i, the set of nodes at maximal distance from k and i are equal so that $Q(k,h+\rho (k)-1) =Q(i,h+\rho (i)-1)$. Similarly, the set of nodes at maximal distance from j and l are equal and $Q(l, h+ \rho (l)-1) = Q(j, h+ \rho (j)-1)$. Because we assume that $n_i \succ n_j$, $Q(i,h+\rho (i)-1) > Q(j,h+\rho (j)-1)$ so that $Q(k, h + \rho (k)-1) > Q(j, h + \rho (j)-1)$, showing that neither $n_k \succ n_l$ nor $n_l \succ n_k$, completing the proof of the Proposition. $\square$

Proof of Proposition 2 (IF)

[IF] Because a regular monotone hierarchy is a monotone hierarchy, we know by Proposition 1 that $i \gtrdot j$ if and only if $n_i \succ n_j$.

Let $d(\ell )$ be the degree of nodes at distance $\ell$ from the root node.

Next we show that the number of geodesic paths of any length d between two nodes is smaller for a node further away from the root. To this end, consider two nodes i and j such that j is a direct successor of i. For any d, if a geodesic path contains j but not i, then i must be an endpoint of the path. Hence, the total number of geodesic paths of length d going through j but not through i is $2p(j,d-1)$. If $d_i \ge 3$, pick a direct successor $k \ne j$ of i, and consider paths of length d connecting successors of k to j. All these paths must go through i and there are $2 p(k,d-1)= 2p(j,d-1)$ such paths. If $d_i=2$, then $d_j \le 2$ so that $2p(j,d-1)=0$ or $2p(j,d-1)=2$. If $2p(j,d-1)=2$, then d is small enough so that there exists at least two paths of length d connecting a node in the network to j through i. Furthermore, if $d = h- \rho (i,i_0)+1$ where h is the number of levels of the hierarchy, there is no path of length d connecting i to a node through j whereas there exist paths of length d connecting a node to j through i, so that $I_i \ge I_j$.

Next, we compute the number of walks emanating from two nodes i and j at different levels of the hierarchy. Let $w_k(d)$ denote the number of walks of length d emanating from a node at level $\ell$. We show that $w_\ell (d) \ge w_{\ell +1}(d)$. We compute the number of walks recursively:

$$\begin{aligned} w_\ell (d)= & {} [d(\ell )-1] w_{\ell +1}(d-1) + w_{\ell -1}(d-1) \text { for } \ell \ge 1 \\ w_0(d)= & {} d(0) w_1(d-1) \end{aligned}$$

We also have $w_\ell (0) = 1$ for all $\ell$ which allows us to start the recursion.

Next we prove that $w_\ell (d) \ge w_{\ell +1}(d)$ for $i=1,..,I-1$ by induction on d The statement is trivially true for all $\ell$ at $d=0$. Now suppose that the statement is true at $d-1$. We first show that the inequality holds for all nodes but the root. For $\ell \ge 1$,

$$\begin{aligned} w_{\ell }(d)= & {} [d(\ell )-1] w_{\ell +1}(d-1) + w_{\ell -1}(d-1) \\\ge & {} [d(\ell +1)-1]w_{\ell +2}(d-1) + w_{\ell }(d-1) \\= & {} w_{\ell +1}(d) \end{aligned}$$

The more difficult step is to show that the statement is also true for the root. To this end, we prove by induction on d that for all $\ell =1,..,h-1$:

$$\begin{aligned} d(0) w_\ell (d) \ge [d(\ell )-1] w_{\ell +1}(d) + w_{\ell -1}(d), \end{aligned}$$

The statement is true at $d=0$ because $d(0)\ge d(\ell )$ for all $\ell \ge 1$. Next compute

$$\begin{aligned} d(0) w_\ell (d)= & {} d(0)[[d(\ell )-1]w_{\ell +1}(d-1) + w_{\ell -1}(d-1)], \\ (d(\ell -1)-1) w_{\ell +1}(d) + w_{\ell -1}(d)= & {} [d(\ell )-1] [[d(\ell +2)-1]w_{\ell +2}(d-1) +w_\ell (d-1)] \\{} & {} + [d(\ell -1)-1] w_\ell (d-1) + w_{\ell -2}(d-1). \end{aligned}$$

By the induction hypothesis,

$$\begin{aligned} d(0) w_{\ell -1}(d-1) \ge [d(\ell -1)-1] w_\ell (d-1) + w_{\ell -2}(d-1), \end{aligned}$$

and

$$\begin{aligned} d(0) w_{\ell +1}(d-1) \ge [d(\ell +1)-1] w_{\ell +2}(d-1) + w_\ell (d-1) \ge [d(\ell +2)-1] w_{\ell +2}(d-1) + w_\ell (d-1). \end{aligned}$$

Replacing, we obtain

$$\begin{aligned} d(0) w_\ell (d) \ge [d(\ell )-1] w_{\ell +1}(d) + w_{\ell -1}(d), \end{aligned}$$

concluding the inductive argument. Applying this formula for $\ell =1$, we have $w_0(d) = d(0) w_1(d-1) \ge [d(1)-1] w_2(d-1) + w_0(d-1) = w_1(d)$, completing the proof that $w_\ell (d) \ge w_{\ell +1}(d)$ for all d.

[ONLY IF] Consider a leaf i of the tree. Then $w_i=(0,0\ldots ,0)$. So all leaves have the same centrality based on the intermediary statistic. They must also have the same centrality based on the neighborhood statistic, which implies that the tree is a regular monotone hierarchy. $\square$

Appendix: simulations—differences in centrality measures by network type

We simulate networks on 40 nodes. We vary the type of network to have three different basic structures, corresponding to a standard random network, a simple version of a stochastic block model, and a variation of a stochastic block model that includes bridge nodes. The first is an Erdos-Renyi random graph in which all links are formed independently. The second is a network that has some homophily: there are two types of nodes and we connect nodes of the same type with a different probability than nodes of different types. The third is a variant of a homophilous network in which some nodes are ‘bridge nodes’ that connect to other nodes with a uniform probability, thus putting them as connector nodes between the two homophilitic groups. We vary the overall average degrees of the networks to be either 2, 5 or 10. In the cases of the homophily and homophily bridge nodes, there are also relative within and across group link probabilities that vary. Given all of these dimensions, we end up with many different networks on which to compare centrality measures.

We then compare 5 different centrality measures on these networks: degree, decay, closeness, diffusion, and Katz–Bonacich. Decay, diffusion and Katz–Bonacich all depend on a parameter that we call the exponential parameter, and we vary that as well.³⁵

The details on the three network types we perform the simulation on are:

• ER random graphs: Each possible link is formed independently with probability $p=\overline{d}/\left( n-1\right)$.

• Homophily:

  There are two equally-sized groups of 20 nodes. Links between pairs of nodes in the same group are formed with probability $p_{same}$ and between pairs of nodes in different groups are formed with probability $p_{diff}$, all independently.

  Letting $p_{same}=H\times p_{diff}$, average degree is:

  $$\begin{aligned} \overline{d} =\left( \frac{n}{2}-1\right) p_{same} +\frac{n}{2}p_{diff} \end{aligned}$$

• Homophily with Bridge Nodes:

  There are L bridge nodes and two equal-sized groups of $\frac{nN-L}{2}$ non-bridge nodes. Each bridge node connects to any other node with probability $p_{b}$. Non-bridge nodes connect to other same group nodes with probability $p_{same}$ and different group nodes with probability $p_{diff}$.

  Letting $p_{same}=H\times p_{diff}$, we set $p_{b} =\overline{d}/\left( n-1\right)$ where $\overline{d}$ is defined as the average degree

  $$\begin{aligned} \overline{d} =\left( \frac{n-L}{2}-1\right) p_{same} +\left( \frac{n-L}{2}\right) p_{diff}+Lp_{b}. \end{aligned}$$

A first thing to note about the simulations (see the tables below) is that the correlation in rankings of the various centrality measures is very high across all of the simulations and measures, often above 0.9, and usually in the 0.8 to 1 range. This is in part reflective of what we have seen from our characterizations: all of these measures operate in a similar manner and are based on nodal statistics that often move in similar ways: nodes with higher degree tend to be closer to other nodes and have more walks to other nodes, and so forth. In terms of differences between measures, closeness and betweenness are more distinguished from the others in terms of correlation, while the other measures all correlate above 0.98 in Table 2.

These extreme correlations are higher than those found in Valente et al. (2008), who also find high correlations, but lower in magnitude, when looking at a series of real data sets.³⁶ The artificial nature of the Erdos-Renyi networks serves as a benchmark from which we can jump off as it results in less differentiation between nodes than one finds in many real-world networks, but also allows us to know that differences among nodes are coming from random variations. As we add homophily in Table 4, and then bridge nodes in Table 5, we see the correlations drop significantly, especially comparing betweenness centrality to the others, and this then has an intuitive interpretation as bridge nodes naturally have high betweenness centrality, but may not stand out according to other measures.

Correlation is a very crude measure, and it does not capture whether nodes are switching ranking or by how much. Some nodes could have dramatically different rankings and yet the correlation could be relatively high overall. Thus, we also look at how many nodes switch rankings between two measures, as well as how the maximal extent to which some node changes rankings. There, we see more substantial differences across centrality measures, and with most measures being more highly distinguished from each other.

As we increase the exponential factor (e.g., from Table 2 to 3), we see greater differences between the measures, as the correlations drop and we see more changes in the rankings. With a very low exponential factor, decay, diffusion, Katz–Bonacich are all very close to degree, while for higher exponential parameters they begin to differentiate themselves. This makes sense as it allows the measures to incorporate information that depends on more of the network, and that is less tied to immediate neighborhoods.

Table 2 Avg degree 2, Erdos Renyi, Decay.15

Table 3 Avg degree 2, Erdos Renyi, Decay.5

Table 4 Avg degree 2, Homophily, Decay.5

Table 5 Avg degree 2, Homophily-Bridge, Decay.5