Source code for nltk.translate.metrics

# Natural Language Toolkit: Translation metrics
#
# Copyright (C) 2001-2018 NLTK Project
# Author: Will Zhang <wilzzha@gmail.com>
#         Guan Gui <ggui@student.unimelb.edu.au>
#         Steven Bird <stevenbird1@gmail.com>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT
from __future__ import division

[docs]def alignment_error_rate(reference, hypothesis, possible=None): """ Return the Alignment Error Rate (AER) of an alignment with respect to a "gold standard" reference alignment. Return an error rate between 0.0 (perfect alignment) and 1.0 (no alignment). >>> from nltk.translate import Alignment >>> ref = Alignment([(0, 0), (1, 1), (2, 2)]) >>> test = Alignment([(0, 0), (1, 2), (2, 1)]) >>> alignment_error_rate(ref, test) # doctest: +ELLIPSIS 0.6666666666666667 :type reference: Alignment :param reference: A gold standard alignment (sure alignments) :type hypothesis: Alignment :param hypothesis: A hypothesis alignment (aka. candidate alignments) :type possible: Alignment or None :param possible: A gold standard reference of possible alignments (defaults to *reference* if None) :rtype: float or None """ if possible is None: possible = reference else: assert(reference.issubset(possible)) # sanity check return (1.0 - (len(hypothesis & reference) + len(hypothesis & possible)) / float(len(hypothesis) + len(reference)))