If one treats all discourse units in the preceding discourse equally, the increase is statistically significant only when a discourse-based coreference system looks back at most four discourse units in order to establish co-referential links.
if one assumes that proximity plays an important role in establishing coreferential links and that referential expressions are more likely to be linked to referees that were used recently in discourse, the increase is statistically significant no matter how many units a discourse-based co-reference system looks back in order to establish co-referential links.