Because the boundaries of the spans annotated in the MPQA corpus are not strictly defined in the annotation guidelines (Wiebe, Wilson, and Cardie 2005), however, measuring precision and recall using exact boundary scoring will result in figures that are too low to be indicative of the usefulness of the system.
most work using this corpus instead use overlap-based precision and recall measures, where a span is counted as correctly detected if it overlaps with a span in the gold standard (Choi, Breck, and Cardie 2006;Breck, Choi, and Cardie 2007).