Query-performance prediction: Setting the expectations straight

The query-performance prediction task has been described as estimating retrieval effectiveness in the absence of relevance judgments. The expectations throughout the years were that improved prediction techniques would translate to improved retrieval approaches. However, this has not yet happened. Herein we provide an in-depth analysis of why this is the case. To this end, we formalize the prediction task in the most general probabilistic terms. Using this formalism we draw novel connections between tasks - and methods used to address these tasks - in federated search, fusion-based retrieval, and query-performance prediction. Furthermore, using formal arguments we show that the ability to estimate the probability of effective retrieval with no relevance judgments (i.e., to predict performance) implies knowledge of how to perform effective retrieval. We also explain why the expectation that using previously proposed query-performance predictors would help to improve retrieval effectiveness was not realized. This is due to a misalignment with the actual goal for which these predictors were devised: ranking queries based on the presumed effectiveness of using them for retrieval over a corpus with a specific retrieval method. Focusing on this specific prediction task, namely query ranking by presumed effectiveness, we present a novel learning-to-rank-based approach that uses Markov Random Fields. The resultant prediction quality substantially transcends that of state-of-the-art predictors.