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