Identifying Cascading Errors using Constraints in Dependency Parsing

Dependency parsers are usually evaluated on attachment accuracy. Whilst easily interpreted, the metric does not illustrate the cascading impact of errors, where the parser chooses an incorrect arc, and is subsequently forced to choose further incorrect arcs elsewhere in the parse. We apply arc-level constraints to MSTparser and ZPar, enforcing the correct analysis of specific error classes, whilst otherwise continuing with decoding. We investigate the direct and indirect impact of applying constraints to the parser. Erroneous NP and punctuation attachments cause the most cascading errors, while incorrect PP and coordination attachments are frequent but less influential. Punctuation is especially challenging, as it has long been ignored in parsing, and serves a variety of disparate syntactic roles.