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From what I understand:

  • a projective dependency parsers can only output projective tree
  • Wikipedia says: "a large majority of sentences in most languages are projective, i.e. they do not contain discontinuities. Only about 15-25% of actual sentences contain a discontinuity"

Therefore, assuming that in English about 15-25% of actual sentences contain a discontinuity, the maximum accuracy (i.e. percentage of correct parse trees for entire sentences) that can achieve a projective dependency parser in English would be 75-85% (since it would fail for all non-projective sentences).

Is this reasoning correct?

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The reasoning is correct, except that parsers' accuracy is never measured in the percentage of correct parse trees for entire sentences. Parsers are evaluated by Labeled Attachment Score (LAS), Unlabeled Attachment Score (UAS), or F1 score, all over individual arcs. See this answer for details.

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