Abstract
We propose indices to measure the difficulty of the named entity (NE) task by looking at test corpora, based on expressions inside and outside the NEs. These indices are intended to estimate the difficulty of each task without actually using an NE system and to be unbiased towards a specific system. The values of the indices are compared with the systems' performance in Japanese documents. We also discuss the difference between NE classes with the indices and show useful clues which will make it easier to recognize NEs.
