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Accounting for Stability of Retrieval Algorithms using Risk-Reward Curves

Past evaluation of information retrieval algorithms has focused largely on achieving good average performance, without much regard for the stability or variance of retrieval results across queries. In fact, two algorithms that superficially appear to have equally desirable average precision performance can have very different stability or risk profiles. A prime example comes from query expansion, where current techniques typically give good average improvements in mean average precision, but are also unstable and have high variance across individual queries [3]. We propose the use of risk-reward curves and related statistics to characterize the tradeoff an algorithm exhibits between a reward property such as mean average precision and a risk property such as the variance of the algorithm – particularly the downside variance, when the algorithm fails or makes performance worse. Such evaluation methods are broadly applicable beyond query expansion to other retrieval operations that must balance risk and reward, such as personalization, document ranking, resource selection, and others.