Differences in effectiveness across sub-collections

The relative performance of retrieval systems when evaluated on one part of a test collection may bear little or no similarity to the relative performance measured on a different part of the collection. In this paper we report the results of a detailed study of the impact that different sub-collections have on retrieval effectiveness, analyzing the effect over many collections, and with different approaches to sub-dividing the collections. The effect is shown to be substantial, impacting on comparisons between retrieval runs that are statistically significant. Some possible causes for the effect are investigated, and the implications of this work are examined for test collection design and for the strength of conclusions one can draw from experimental results.