This evaluation measure accounts for improvements in coverage, i.e., it rewards cases where answers are found for queries that did not have an adequate or material answer before.
the mean reciprocal rank (MRR) measure standardly used in QA can have the effect of preferring systems that find answers only for a small set of queries, but rank them higher than systems with  Table 4: Examples for queries and expansion terms yielding improved (+), decreased (-), or unchanged (0) retrieval performance compared to retrieval without expansion.