The effect of pool depth on system evaluation in TREC

The TREC benchmarking exercise for information retrieval (IR) experiments has provided a forum and an opportunity for IR researchers to evaluate the performance of their approaches to the IR task and has resulted in improvements in IR effectiveness. Typically, retrieval performance has been measured in terms of precision and recall, and comparisons between different IR approaches have been based on these measures. These measures are in turn dependent on the so-called “pool depth” used to discover relevant documents. Whereas there is evidence to suggest that the pool depth size used for TREC evaluations adequately identifies the relevant documents in the entire test data collection, we consider how it affects the evaluations of individual systems. The data used comes from the Sixth TREC conference, TREC-6. By fitting appropriate regression models we explore whether different pool depths confer advantages or disadvantages on different retrieval systems when they are compared. As a consequence of this model fitting, a pair of measures for each retrieval run, which are related to precision and recall, emerge. For each system, these give an extrapolation for the number of relevant documents the system would have been deemed to have retrieved if an indefinitely large pool size had been used, and also a measure of the sensitivity of each system to pool size. We concur that even on the basis of analyses of individual systems, the pool depth of 100 used by TREC is adequate.