Note that some of the works discussed here transfer knowledge from external datasets into the QA task they address (Chung et al., 2017;Pan et al., 2019;Min et al., 2017;Qiu et al., 2018;Chen et al., 2017).
the improvements are not large, e.g., the maximum improvement is 1.6% (when k = 4), which indicates that ROCC is robust to a certain extent to lexical variation.