# Minimal test collections for low-cost evaluation of Audio Music Similarity and Retrieval systems

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## Abstract

Reliable evaluation of Information Retrieval systems requires large amounts of relevance judgments. Making these annotations is not only tedious but also complex for many Music Information Retrieval tasks. As a result, performing such evaluations usually requires too much effort. A low-cost alternative is the application of Minimal Test Collections algorithms, which offer very reliable results while significantly reducing the required annotation effort. The idea is to represent effectiveness scores as random variables that can be estimated, iteratively selecting which documents to judge so that we can compute accurate estimates with a certain degree of confidence and with the least effort. In this paper we show the application of Minimal Test Collections to the evaluation of the Audio Music Similarity and Retrieval task, run by the annual MIREX evaluation campaign. An analysis with the MIREX 2007, 2009, 2010 and 2011 data shows that with as little as 2 % of the total judgments we can obtain accurate estimates of the ranking of systems. We also present a method to rank systems without making any annotations, which can be successfully used when little or no resources are available.

### Keywords

Music information retrieval Evaluation Experimentation Test collections Relevance judgments## 1 Introduction

The evaluation of Information Retrieval (IR) systems requires a test collection, usually containing a set of documents, a set of task-specific queries, and a set of annotations that provide information as to what results a system should return for each query [10, 22]. Depending on the task, the set of queries may comprise the collection of documents itself, and the type of annotations can differ widely. In the field of Music IR (MIR), building these collections is very problematic due to the very nature of the musical information, legal restrictions upon the documents, etc. [7]. In addition, annotating a test collection is a very time-consuming and expensive process for some MIR tasks. For instance, annotating a single clip for Audio Melody Extraction can take several hours. As a result, test collections for MIR tasks use to be very small, biased, and unlikely to change from year to year, posing serious problems for the proper evolution of the field [17].

The annual Music Information Retrieval Evaluation eXchange (MIREX) started in 2005 as an international forum to promote and perform evaluation of MIR systems for various tasks [8]. MIREX was developed following the principles and methodologies that have made the Text REtrieval Conference (TREC) [24] such a successful forum for evaluating Text IR systems [6, 23]. However, since its inception in 2005, the MIREX campaigns have evolved in parallel to TREC, practically ignoring all recent developments in the evaluation of IR systems [10, 17]. In fact, the last 5 years have witnessed several works on low-cost, yet reliable evaluation techniques, allowing the number of queries used to grow up to as many as 40,000 [5]. One of these works is the development of algorithms for evaluation with Minimal Test Collections (MTC) [1, 2, 3].

Summary of MIREX AMS editions

Year | Teams | Systems | Queries | Results | Judgments | Overlap |
---|---|---|---|---|---|---|

2006 | 5 | 6 | 60 | 1,800 | \(3{\times }1{,}629\) | 10 % |

2007 | 8 | 12 | 100 | 6,000 | 4,832 | 19 % |

2009 | 9 | 15 | 100 | 7,500 | 6,732 | 10 % |

2010 | 5 | 8 | 100 | 4,000 | 2,737 | 32 % |

2011 | 10 | 18 | 100 | 9,000 | 6,322 | 30 % |

Each edition of the AMS task requires the work of dozens of volunteers to perform similarity judgments, telling how similar two 30 s audio clips are. In the last edition, in 2011, 6,322 of these judgments were needed, meaning that at least 53 h of assessor time were needed to complete the judging task. In practice, though, collecting all these judgments takes several days, even weeks [11]. But along with the Symbolic Melodic Similarity (SMS) task, AMS is one of the couple of exceptions for which a new set of queries and relevance judgments are put together every year. Most of the MIR tasks just use the same collections over and over again because they are too expensive to build, especially in terms of judging or annotation effort. Therefore, the study of low-cost evaluation methodologies is imperative for the development of proper test collections to reliably evaluate MIR systems and properly advance the state of the art [17].

Developing low-cost evaluation methodologies is essential for private, in-house evaluations too. A researcher investigating several improvements of an existing MIR technique is not really interested in knowing how well they perform for the task (which is highly dependent on the test collection anyway), but in which one performs better. That is, she is interested in the *comparative* evaluation of systems. MTC is specifically designed for these cases: it minimizes the annotation effort needed to find a difference between systems, iteratively selecting for judging those documents that are more informative to figure out the difference between systems, and reusing previous judgments when available.

## 2 AMS evaluation

The gain of a document is a measure of how much information the user will gain from inspecting that result. In MIREX, there are two different scales [11, 19]: the Broad scale is a 3-point graded scale where a document is considered either not similar to the query (gain 0), somewhat similar (gain 1) or very similar (gain 2); and the Fine scale, where the gain of a document ranges from 0 (not similar at all) to 100 (identical to the query)^{1}. These gain scores are assigned by humans, who make similarity judgments between queries and documents. After all the judging is done, every system gets an \(AG@k\) score for each query, and then they are ranked by their mean score across all queries.

To minimize random effects due to the particular sample of queries chosen, the Friedman test is run with the Average Gain scores of every system to look for significant differences, and the Tukey’s HSD test is then used to correct the experiment-wide Type I error rate [19]. The grand results of the evaluation are therefore scale-dependent pairwise comparisons between systems, telling which one is better for the current set of queries \(\mathcal Q ,\) and whether the observed difference was found to be statistically significant.

## 3 Evaluation with incomplete judgments

The evaluation methodology used in MIREX is expensive in the sense that a complete set of similarity judgments is needed: the top \(k\) documents retrieved by every system have to be judged for every query. However, we may investigate how to compare systems so that we do not need to judge all documents and still be confident about the result of an evaluation experiment.

The idea is to use random variables to represent gain scores. The upside is that their value can be estimated fairly well for most documents; the downside is that these estimates will have some degree of uncertainty. The goal of MTC is to select for judging those documents that allow us to compute good estimates of the difference between systems with very few judgments.

### 3.1 \(AG@k\) as a random variable

### 3.2 Difference in \(AG@k\)

^{2}:

^{3}[8, 19], queries are independent of each other, so the expectation and variance are:

### 3.3 Distribution of \(\Delta AG@k\)

To compute the confidence in the sign, we need to know the distribution of \(\overline{\Delta AG@k}.\) For a relevance scale with only two levels (similar and not similar), \(AG@k\) is basically the same as \(P@k\) (precision at \(k\)), which can be approximated by a normal distribution under a binomial or uniform prior distribution of \(G_i\) [2]. In our case, the Broad scale has 3 possible levels, and the Fine scale has 101 levels.

But \(AG@k\) turns out to be a special case. Let \(G\) be a random variable representing the gain of the top \(k\) documents retrieved by a system for all possible queries, and let the set \(\{AG@k_1,\ldots ,AG@k_{|\mathcal Q |}\}\) be a random sample of size \(|\mathcal Q |\) where each \(AG@k_q\) is the average gain of \(k\) documents sampled from \(G.\) By the Central Limit Theorem, as \(|\mathcal Q |\rightarrow \infty \) the distribution of the sample mean \(\overline{AG@k}=\sum {AG@k_q / |\mathcal Q |}\) approximates a normal distribution, regardless of the underlying distribution of \(G.\) Therefore, with a large number of queries \(\overline{\Delta AG@k}\) can be approximated by a normal distribution, because it is the sum of two variables approximately normal themselves.

### 3.4 Document selection

Equations (4) and (5) can be used to estimate the difference between two systems with an incomplete set of judgments, but the problem is: which documents should we judge? Ideally, we want to judge only those that are most informative to know the sign of the difference in \(AG@k.\) For just two systems it is obvious from Eq. (3) only documents retrieved by one system but not by the other one are informative. For an arbitrary number of queries, we can just refer to a query-document pair as a single document (i.e. the gain of a document *for* a particular query).

For the stopping condition we compute the mean confidence across all system pairs: if it is sufficiently large, we stop judging altogether. We call this the *confidence in the ranking*. We note though that MTC can be used with a different stopping condition. For instance, we may require *at least* 95 % confidence in *all* comparisons, as opposed to an *average* of 95 % as we do here. In such cases, the definition of \(w_i\) could differ from that in Eq. (8). For instance, we could consider just the system pairs for which \(C_\mathsf{AB }<1-\alpha ,\) and make their contribution to \(w_i\) proportional to \(C_\mathsf{AB }.\) We could further modify the algorithm by considering the magnitude of the difference between systems instead of just its sign [18]. This would allow us to estimate system differences from the perspective of expected user satisfaction, for instance by computing \(P\left(\overline{\Delta AG@k}\le -0.3\right)\) instead of \(P\left(\overline{\Delta AG@k}\le 0\right).\)

## 4 Estimation of gain scores

Equations (6) and (7) allow us to compute the confidence in the sign of the difference between two systems. But tracking back to Eq. (1), we still need to know what the distribution of \(G_i\) is; that is, what \(P(G_i=l)\) is for each of the labels in the similarity scale used. There are two immediate choices: a fixed distribution for each document \(i,\) maybe estimated from judgments in previous MIREX editions; or a distribution for each document as returned by a model fitted with various features.

### 4.1 Distribution of gain scores

A simple choice is to assume that every similarity assignment is equally likely [3, 20]. For the Broad scale, all three assignments would have probability \(1/3,\) while for the Fine scale each assignment would have probability \(1/101.\) According to Eq. (1), an arbitrary unjudged document would have expectation 1 and variance \(2/3\) in the Broad scale, and in the Fine scale it would have expectation 50 and variance 850.

A better alternative is to estimate the gain score of each document individually [1, 2, 4]. The problem reduces then to fitting a model that, given certain features about a query-document, allows us to estimate its gain score. We may consider two frameworks for creating such a model: classification and regression. The classification approach is not appropriate because it ignores the order of the labels. In the Broad scale, for instance, it means that if the true gain of a document were 0, an estimation of 1 would be as good as an estimation of 2, while the latter is clearly worse. Linear regression is not appropriate either, because the predicted gains could be well outside the limits [0–2] and [0–100]. This could be solved with truncated regression [13], but we would still need to make assumptions about its underlying distribution. Multinomial regression has the same problem as classification, namely that it ignores the order of the levels in the outcome.

^{4}:

Therefore, the ordinal logistic framework allows us to estimate the distribution \(P(G_i=l)\) in Eq. (1), which in turn enables the computation of expectation and variance as usual. As opposed to using the uniform distribution, this model is expected to produce estimates closer to the true score and with reduced variance. As a result, the confidence calculations as per Eq. (7) are expected to be more reliable and require fewer judgments to pass a threshold like 95 %.

### 4.2 Features used and fitted models

We consider two types of features to use in the above model in order to estimate gain scores: output-based features and judgment-based features.

#### 4.2.1 Output-based features

*pSYS*: percentage of systems that retrieved \(d\) for \(q.\) Intuitively, the more systems retrieve \(d,\) the more likely for it to be similar to \(q.\)*pTEAM*: percentage of research teams participating in MIREX that retrieved \(d\) for \(q.\) Systems by the same team are likely to return similar documents, so the effect of*pSYS*could be biased if teams participate with a large number of systems.*pTEAM*can be used to reduce this bias.*OV*: degree of overlap between systems, to calibrate inherent similarities among systems when using the*pSYS*and*pTEAM*features.*aRANK*: average rank at which systems retrieved \(d\) for \(q.\) Documents retrieved closer to the top of the results lists are expected to be more similar to \(q.\)*sGEN*: whether the musical genre of \(d\) is the same as \(q\)’s (either 1 or 0), as documents of the same genre are usually considered similar to each other [14].*pGEN*: percentage of all documents retrieved for \(q\) that belong to the same musical genre as \(d\) does.*pART*: percentage of all documents retrieved for \(q\) that belong to the same artist as \(d\) does. Note that a feature like*sGEN*for artists does not make sense because all retrieved documents by \(q\)’s artist are filtered out [8, 9].

#### 4.2.2 Judgment-based features

*aSYS*: average gain score obtained by the systems that retrieved \(d\) for \(q.\) Intuitively, a document retrieved by good systems is likely to be a good result.*aDOC*: average gain score of all the other documents retrieved for \(q.\) Likewise, this feature models query difficulty: if documents retrieved for \(q\) are not similar, \(d\) is not likely to be similar either.*aGEN*: average gain score of the documents retrieved for \(q\) that belong to the same genre as \(d\) does.*aART*: average gain score of the documents retrieved for \(q\) and by the same artist as \(d\)’s.

#### 4.2.3 Fitted models

We used data from the MIREX 2007, 2009, 2010 and 2011 editions of the Audio Music Similarity and Retrieval task to fit the models following the regression framework described in Sect. 4.1. Starting with a saturated model, we simplified to a model, called \(L_{\mathrm{judge}},\) using the features *pTEAM*, *OV*, *aSYS* and *aART*. All these features showed a very significant effect on the response (\(p<0.0001\)). While other features did improve the model, they did so very marginally, so we decided to keep it as simple as possible. The coefficient of determination \(R^2\) can be used to assess the goodness of fit, measuring the proportion of variability in the outcome that is accounted for by the model. The predictions of \(L_{\mathrm{judge}}\) are particularly good, with an adjusted \(R^2\) score of approximately 0.9 (the value \(R^2=1\) means that the model offers a perfect fit of the data).

Even though \(L_{\mathrm{judge}}\) produces very good results, we can only use it to estimate the \(G_i\) scores of documents for which we can compute both *aSYS* and *aART*. However, because our goal is to reduce the amount of judging as much as possible, we will not be able to estimate the gain scores for most of the documents until we have made a fair amount of judgments. Therefore, we decided to fit another model, called \(L_{\mathrm{output}},\) that only uses output-based features. With this model, we can always estimate \(G_i\) scores, even when there are no judgments available at all.

Proceeding as before, we simplified to a model using the features *pTEAM*, *OV*, *pART*, *sGEN*, *pGEN* and the *sGEN:pGEN* interaction. Despite all features showed again a significant effect (\(p<0.0001\)), the predictions were significantly worse than with \(L_{\mathrm{judge}},\) resulting in an adjusted \(R^2\) score of approximately 0.35.

When fitting the models for the Fine scale, we further simplified by breaking the scale down to 10 levels rather than the original 101. Therefore, we actually use the scale \(\{0, 11, 22,\ldots , 99\}.\) In order to avoid overfitting, when estimating the gain scores for one MIREX edition we excluded all data from that edition when fitting the model. Therefore, we actually fitted \(L_{\mathrm{judge}}\) and \(L_{\mathrm{output}}\) for each scale and each edition. See the appendix for more details regarding the models.

### 4.3 Estimation errors in practice

Average error and variance of the \(G_i\) estimates computed with the uniform distribution and regression models

Year | Broad scale | Fine scale | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Uniform | \(L_{\mathrm{output}}\) | \(L_{\mathrm{judge}}\) | Uniform | \(L_{\mathrm{output}}\) | \(L_{\mathrm{judge}}\) | |||||||

RMSE | Var | RMSE | Var | RMSE | Var | RMSE | Var | RMSE | Var | RMSE | Var | |

2007 | 0.813 | 0.667 | 0.639 | 0.436 | 0.260 | 0.067 | 31.9 | 850 | 24.3 | 601 | 8.83 | 70 |

2009 | 0.812 | 0.667 | 0.632 | 0.454 | 0.254 | 0.069 | 31.1 | 850 | 23.4 | 626 | 8.76 | 73 |

2010 | 0.794 | 0.667 | 0.706 | 0.394 | 0.283 | 0.07 | 30.2 | 850 | 26.1 | 549 | 8.94 | 73 |

2011 | 0.789 | 0.667 | 0.690 | 0.390 | 0.304 | 0.078 | 29.6 | 850 | 25.2 | 561 | 9.36 | 72 |

In MIREX 2006 three different assessors provided judgments for each query-document pair [8, 11]. If we consider one assessor’s judgments as the truth, and the other’s as mere estimates, we find that the average RMSE among assessors was 0.795 with the Broad scale and 31.2 with the Fine scale. We note that these errors are extremely similar to the errors of the \(L_{\mathrm{output}}\) model (see Table 2), and quite larger than the errors of the \(L_{\mathrm{judge}}\) model. Therefore, we argue that the errors we make when using MTC or ranking without judgments are comparable to the differences we should expect just by having a different human assessor in the first place [11, 21]. The MIREX evaluations assume arbitrary final users, so these errors can be ignored for all practical purposes. If no arbitrary users were assumed, but specific users were considered for instance in personalization [18], then our estimates would be erroneous to the degree reported here.

We also compared the average variance of the estimates. In Sect. 4.1 we saw that the variance in the uniform estimates is \(2/3\) with the Broad scale and 850 with the Fine scale. As Table 2 shows, the regression models improve the estimates also in terms of variance. The \(L_{\mathrm{judge}}\) model reduces variance by one order of magnitude: \(\approx \!\!0.07\) with Broad judgments and \(\approx \!72\) with Fine judgments. Thus, the regression models provide better estimates and reduce variance to achieve high confidence in the sign differences earlier in the process.

## 5 Results

We simulated the use of MTC to evaluate all systems from the MIREX 2007, 2009, 2010 and 2011 Audio Music Similarity and Retrieval task (see Table 1). The number of pairwise system comparisons are 66, 105, 28 and 153, respectively. Recall that the \(L_{\mathrm{output}}\) and \(L_{\mathrm{judge}}\) models for one edition are fitted ignoring all information from that same edition, thus avoiding overfitting. When using MTC with the regression models, all \(G_i\) scores are estimated at the beginning with \(L_{\mathrm{output}},\) and updated every 20 judgments, when possible, with \(L_{\mathrm{judge}}.\)

Judgments needed by MTC to reach 95 % confidence in the ranking of systems and accuracy of the sign estimates at that point

Year | Total judgments | Broad scale | Fine scale | ||||
---|---|---|---|---|---|---|---|

Judgments | Accuracy | \(\tau \) | Judgments | Accuracy | \(\tau \) | ||

2007 | 4,832 | 200 (4.1 %) | 0.955 | 0.909 | 80 (1.7 %) | 0.955 | 0.909 |

2009 | 6,732 | 300 (4.5 %) | 0.971 | 0.943 | 440 (6.5 %) | 0.952 | 0.905 |

2010 | 2,737 | 13 (0.5 %) | 0.893 | 0.786 | 2 (0.1 %) | 0.857 | 0.714 |

2011 | 6,322 | 120 (1.9 %) | 0.941 | 0.882 | 120 (1.9 %) | 0.941 | 0.882 |

### 5.1 Accuracy of the individual estimates

Despite the average confidence in the ranking generally corresponds to the average accuracy of the sign estimates, there can be the case where the average confidence is biased by a few comparisons for which we are extremely confident. The question now is: how trustworthy are each of the individual estimates? We ran MTC with all four collections and the two similarity scales, and stopped judging when the average confidence was at least 95 %. The 352 system pairs from all four collections were divided by confidence in the sign of the individual \(E\left[\overline{\Delta AG@k}\right].\)

Accuracy versus confidence in the sign estimates when running MTC to 95 % confidence in the ranking

Conf. | Broad scale | Fine scale | ||
---|---|---|---|---|

In bin | Acc. | In bin | Acc. | |

[0.50, 0.60) | 7 (2.0 %) | 0.714 | 13 (3.7 %) | 0.615 |

[0.60, 0.70) | 15 (4.3 %) | 0.733 | 13 (3.7 %) | 0.846 |

[0.70, 0.80) | 11 (3.1 %) | 0.818 | 7 (2.0 %) | 0.714 |

[0.80, 0.90) | 24 (6.8 %) | 0.833 | 24 (6.8 %) | 0.833 |

[0.90, 0.95) | 15 (4.3 %) | 0.733 | 15 (4.3 %) | 0.667 |

[0.95, 0.99) | 31 (8.8 %) | 1.000 | 22 (6.2 %) | 0.909 |

[0.99, 1) | 249 (70.7 %) | 0.992 | 258 (73.3 %) | 0.996 |

### 5.2 Ranking systems without judgments

As discussed above, the confidence in the ranking is quite high with very few judgments, so next we ask the question: how well can we rank systems *with no judgments at all*? Soboroff et al. [16] first studied this problem with systems submitted to TREC, showing that randomly considering documents as relevant correlated positively with the true TREC rankings. Rather than using random judgments, we use the estimates provided by the \(L_{\mathrm{output}}\) regression model. Note that the \(L_{\mathrm{judge}}\) model cannot be used because it does require some known judgments.

Confidence and accuracy of the estimated ranking when no judgments are made

Year | Broad scale | Fine scale | ||||
---|---|---|---|---|---|---|

Conf. | Acc. | \(\tau \) | Conf. | Acc. | \(\tau \) | |

2007 | 0.941 | 0.909 | 0.818 | 0.946 | 0.924 | 0.848 |

2009 | 0.925 | 0.933 | 0.867 | 0.929 | 0.943 | 0.886 |

2010 | 0.947 | 0.893 | 0.786 | 0.949 | 0.857 | 0.714 |

2011 | 0.939 | 0.948 | 0.895 | 0.942 | 0.948 | 0.895 |

Accuracy versus confidence in the sign estimates when ranking systems in all collections and with no judgments

Conf. | Broad scale | Fine scale | ||
---|---|---|---|---|

In bin | Acc. | In bin | Acc. | |

[0.50, 0.60) | 16 (4.5 %) | 0.500 | 16 (4.5 %) | 0.625 |

[0.60, 0.70) | 17 (4.8 %) | 0.882 | 15 (4.3 %) | 0.867 |

[0.70, 0.80) | 15 (4.3 %) | 0.800 | 15 (4.3 %) | 0.733 |

[0.80, 0.90) | 24 (6.8 %) | 0.792 | 24 (6.8 %) | 0.792 |

[0.90, 0.95) | 16 (4.5 %) | 0.875 | 13 (3.7 %) | 0.846 |

[0.95, 0.99) | 33 (9.4 %) | 0.909 | 31 (8.8 %) | 0.903 |

[0.99, 1) | 231 (65.6 %) | 0.996 | 238 (67.6 %) | 0.996 |

## 6 Conclusions

We have shown how to adapt the Minimal Test Collections (MTC) family of algorithms for the evaluation of the MIREX Audio Music Similarity and Retrieval task. We showed that the distribution of \(\overline{AG@k}\) scores is normally distributed, which allows us to look at it as a random variable whose expectation may be estimated with a certain level of confidence. This confidence is proportional to the number of similarity judgments available, and MTC ensures that the set of judgments we make to reach some confidence level is minimal.

Using data from the previous MIREX AMS evaluations, we fitted a model that allows us to predict gain scores when no judgments are available, and another model that considerably improves the predictions when judgments are available. Aided by these two models, MTC is shown to dramatically reduce the judging effort needed to rank systems with 95 % confidence. We simulated the MIREX AMS evaluations from 2007, 2009, 2010 and 2011, and showed that the average number of judgments needed is just 3 % with the Broad scale and 1.8 % with the Fine scale. The average accuracy of the estimated rankings is 0.948 with the Broad scale and 0.947 with the Fine scale, showing that MTC coupled with our models does not only require very little effort, but also produces accurate estimates. In fact, when systems show a statistically significant difference our estimates are always correct.

We further showed that these models can be used to rank systems without the need of making any judgments at all. Even though overall accuracy is slightly lower than when running MTC, we showed that the individual confidence scores can be trusted. Also, we showed that *the estimation errors are negligible in practice, because they compare to the disagreements produced by different human assessors*. This method can thus be employed to quickly check if there is a substantial difference between systems.

In general, the Fine scale seems to require fewer judgments than the Broad scale, while at the same time produces similarly accurate estimates. In previous work we also showed that the Fine scale is slightly more powerful and similarly stable as the Broad scale for a variety of measures [19], and that it is better correlated with final user satisfaction too [18]. Therefore, the evidence so far seems to indicate that the Fine scale works better than the Broad scale, suggesting its use alone in the MIREX AMS evaluations. Dropping the Broad scale would also lower the cost of the evaluations, at least in terms of judging time.

## 7 Future work

Two clear lines for future work can be identified. In this paper we used two sets of features to fit the regression models that allow us to predict gain scores: features based on the output of the systems and metadata, as well as features based on the known judgments. While these features work well in practice, a third set of features to consider could take advantage of the actual musical content used in the test collections, such as the similarity between the current document and those that have been judged as highly similar to the query. Unfortunately, the collection used in MIREX is not public, so we were not able to study these features here. Nonetheless, further research should definitely explore this line. Also, by no means are our models the only ones possible; other features or frameworks might prove better to predict gain scores. For instance, trying to predict gain scores on a per-system or per-query basis would probably improve the results.

The most important direction for further research is the study of low-cost evaluation methodologies for other MIR tasks. In accordance with previous work [19], we have shown here that the effort in evaluating a set of AMS systems can be greatly reduced, leaving open the possibility of building brand new test collections for other tasks for which making annotations is very expensive. For instance, the group of volunteers requested by MIREX for the annual evaluation of the AMS and SMS tasks could probably be better employed if some of them were instead dedicated to incrementally add new annotations for the other tasks in clear need of new collections [15].

Another clear setting for the application of low-cost methodologies is that of a researcher evaluating a set of systems with a private document collection, a scenario very common in MIR given the legal restrictions when sharing music corpora [7]. Those researchers, and in most cases public forums too, do not have the possibility of requesting large pools of external volunteers for annotating their collections. Thus, being able to evaluate systems with the minimal effort is paramount. To this end, low-cost evaluation methodologies must be investigated for the wealth of MIR tasks.

But in most of these tasks researchers rely on test collections annotated *a priori*, which can be very expensive and time consuming to build. However, we have seen that not all annotations are necessary to accurately rank systems. For instance, if two Audio Melody Extraction algorithms predict the same F0 (fundamental frequency) in a given audio frame, whether that F0 prediction is correct or not is not useful to know which of the two systems is better. The adoption of *a posteriori* evaluation methodologies such as MTC can take advantage of this idea to greatly reduce the annotation cost or allow the use of significantly larger collections. Getting to that point, though, requires a shift in the current evaluation practices. But given the benefits of doing so, both in terms of cost and reliability, we strongly encourage the MIR community to study these evaluation alternatives and progressively adopt them for a more rapid and stable development of the field.

## Footnotes

- 1.
In early editions of MIREX it was defined from 0 to 10, with one decimal digit. Both definitions are equivalent.

- 2.
The indicator functions are squared in the variance so all documents have a positive contribution to the total variance.

- 3.
Note that this is rarely true in Text Information Retrieval.

- 4.
Note that \(P(G_i\ge l_1|f_i)\) is always 1.

## Notes

### Acknowledgments

This research was supported by the Spanish Government (TSI-020110-2009-439, HAR2011-27540) as well as the Austrian Science Funds (FWF): P22856-N23.

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