Repeatability Corner Cases in Document Ranking: The Impact of Score Ties
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Document ranking experiments should be repeatable: running the same ranking model over the same collection with the same queries should yield exactly the same output. However, the presence of different documents with the same score may yield non-deterministic rankings, making repeatability not as trivial as one might imagine. In the context of our work using the open-source Lucene search engine, score ties are broken by internal document ids, which are assigned at index time. Due to multi-threaded indexing, which makes experimentation with large modern document collections practical, internal document ids are not assigned consistently between different index instances of the same collection, and thus score ties are broken unpredictably. This short paper examines the effectiveness impact of such score ties, quantifying the variability that can be attributed to this phenomenon. The obvious solution to this non-determinism and to ensure repeatable document ranking is to break score ties using external collection document ids. This approach, however, comes with measurable efficiency costs due to the necessity of consulting external identifiers during the inner loop of query evaluation.
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