Predicting the performance of linearly combined IR systems

We introduce a new technique for analyzing combination models. The technique allows us to make qualitative conclusions about which IR systems should be combined. We achieve this by using a linear regression to accurately (T ' = 0.98) predict the performance of the combined system based on quantitative measurements of individual component systems taken from TREC5. When applied to a linear model (weighted sum of relevance scores), the technique supports several previously suggested hypotheses: one should maximize both the individual systems' performances and the overlap of relevant documents between systems, while minimizing the overlap of nonrelevant documents. It also suggests new conclusions: both systems should distribute scores similarly, but not rank relevant documents similarly. It furthermore suggests that the linear model is only able to exploit a fraction of the benefit possible from combination. The technique is general in nature and capable of pointing out the strengths and weaknesses of any given combination approach.