Some evaluation data may contain anaphors which are more difficult to resolve, such as anaphors that are (slightly) ambiguous and require real-world knowledge for their resolution, or anaphors that have a high number of competing candidates, or that have their antecedents far away both in terms of sentences/clauses and in terms of number of "intervening" NPs etc.
we suggest that in addition to the evaluation results, information should be provided in the evaluation data as to how difficult the anaphors are to resolve.