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| # Natural Language Toolkit: Translation metrics | |
| # | |
| # Copyright (C) 2001-2023 NLTK Project | |
| # Author: Will Zhang <wilzzha@gmail.com> | |
| # Guan Gui <ggui@student.unimelb.edu.au> | |
| # Steven Bird <stevenbird1@gmail.com> | |
| # 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) | |
| ) | |