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# Natural Language Toolkit: Translation metrics
#
# Copyright (C) 2001-2023 NLTK Project
# Author: Will Zhang <[email protected]>
# Guan Gui <[email protected]>
# Steven Bird <[email protected]>
# URL: <https://www.nltk.org/>
# For license information, see LICENSE.TXT
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)
)
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