  Conversational passage retrieval relies on question rewriting to modify the
original question so that it no longer depends on the conversation history.
Several methods for question rewriting have recently been proposed, but they
were compared under different retrieval pipelines. We bridge this gap by
thoroughly evaluating those question rewriting methods on the TREC CAsT 2019
and 2020 datasets under the same retrieval pipeline. We analyze the effect of
different types of question rewriting methods on retrieval performance and show
that by combining question rewriting methods of different types we can achieve
state-of-the-art performance on both datasets.
