Clarity re-visited.

We present a novel interpretation of Clarity [5], a widely used query performance predictor. While Clarity is commonly described as a measure of the "distance" between the language model of the top-retrieved documents and that of the collection, we show that it actually quantifies an additional property of the result list, namely, its diversity. This analysis, along with empirical evaluation, helps to explain the low prediction quality of Clarity for large-scale Web collections.