.. Copyright (C) 2001-2010 NLTK Project
.. For license information, see LICENSE.TXT

=======
Metrics
=======

The `nltk.metrics` package provides a variety of *evaluation measures*
which can be used for a wide variety of NLP tasks.

   >>> from nltk.metrics import *

------------------
Standard IR Scores
------------------

We can use standard scores from information retrieval to test the
performance of taggers, chunkers, etc.

    >>> reference = 'DET NN VB DET JJ NN NN IN DET NN'.split()
    >>> test    = 'DET VB VB DET NN NN NN IN DET NN'.split()
    >>> accuracy(reference, test)
    0.80000000000000004
    

The following measures apply to sets:

    >>> reference_set = set(reference)
    >>> test_set = set(test)
    >>> precision(reference_set, test_set)
    1.0
    >>> recall(reference_set, test_set)
    0.80000000000000004
    >>> f_measure(reference_set, test_set)
    0.88888888888888884

Measuring the likelihood of the data, given probability distributions:

    >>> from nltk import FreqDist, MLEProbDist
    >>> pdist1 = MLEProbDist(FreqDist("aldjfalskfjaldsf"))
    >>> pdist2 = MLEProbDist(FreqDist("aldjfalssjjlldss"))
    >>> log_likelihood(['a', 'd'], [pdist1, pdist2])
    -2.707518749639422


----------------
Distance Metrics
----------------

String edit distance (Levenshtein):

    >>> edit_distance("rain", "shine")
    3

Other distance measures:

    >>> s1 = set([1,2,3,4])
    >>> s2 = set([3,4,5])
    >>> binary_distance(s1, s2)
    1.0
    >>> jaccard_distance(s1, s2)
    0.59999999999999998
    >>> masi_distance(s1, s2)
    0.5

----------------------
Miscellaneous Measures
----------------------

Rank Correlation works with two dictionaries mapping keys to ranks.  The dictionaries should have the same set of keys.

    >>> spearman_correlation({'e':1, 't':2, 'a':3}, {'e':1, 'a':2, 't':3})
    0.5

Windowdiff uses a sliding window in comparing two segmentations of the same input (e.g. tokenizations, chunkings).
Segmentations are represented using strings of zeros and ones.

    >>> s1 = "00000010000000001000000"
    >>> s2 = "00000001000000010000000"
    >>> s3 = "00010000000000000001000"
    >>> windowdiff(s1, s1, 3)
    0
    >>> windowdiff(s1, s2, 3)
    4
    >>> windowdiff(s2, s3, 3)
    16

----------------
Confusion Matrix
----------------

    >>> reference = 'This is the reference data.  Testing 123.  aoaeoeoe'
    >>> test =      'Thos iz_the rifirenci data.  Testeng 123.  aoaeoeoe'
    >>> print ConfusionMatrix(reference, test)
      |   . 1 2 3 T _ a c d e f g h i n o r s t z |
    --+-------------------------------------------+
      |<8>. . . . . 1 . . . . . . . . . . . . . . |
    . | .<2>. . . . . . . . . . . . . . . . . . . |
    1 | . .<1>. . . . . . . . . . . . . . . . . . |
    2 | . . .<1>. . . . . . . . . . . . . . . . . |
    3 | . . . .<1>. . . . . . . . . . . . . . . . |
    T | . . . . .<2>. . . . . . . . . . . . . . . |
    _ | . . . . . .<.>. . . . . . . . . . . . . . |
    a | . . . . . . .<4>. . . . . . . . . . . . . |
    c | . . . . . . . .<1>. . . . . . . . . . . . |
    d | . . . . . . . . .<1>. . . . . . . . . . . |
    e | . . . . . . . . . .<6>. . . 3 . . . . . . |
    f | . . . . . . . . . . .<1>. . . . . . . . . |
    g | . . . . . . . . . . . .<1>. . . . . . . . |
    h | . . . . . . . . . . . . .<2>. . . . . . . |
    i | . . . . . . . . . . 1 . . .<1>. 1 . . . . |
    n | . . . . . . . . . . . . . . .<2>. . . . . |
    o | . . . . . . . . . . . . . . . .<3>. . . . |
    r | . . . . . . . . . . . . . . . . .<2>. . . |
    s | . . . . . . . . . . . . . . . . . .<2>. 1 |
    t | . . . . . . . . . . . . . . . . . . .<3>. |
    z | . . . . . . . . . . . . . . . . . . . .<.>|
    --+-------------------------------------------+
    (row = reference; col = test)
    <BLANKLINE>
    
    >>> cm = ConfusionMatrix(reference, test)
    >>> print cm.pp(sort_by_count=True)
      |   e a i o s t . T h n r 1 2 3 c d f g _ z |
    --+-------------------------------------------+
      |<8>. . . . . . . . . . . . . . . . . . 1 . |
    e | .<6>. 3 . . . . . . . . . . . . . . . . . |
    a | . .<4>. . . . . . . . . . . . . . . . . . |
    i | . 1 .<1>1 . . . . . . . . . . . . . . . . |
    o | . . . .<3>. . . . . . . . . . . . . . . . |
    s | . . . . .<2>. . . . . . . . . . . . . . 1 |
    t | . . . . . .<3>. . . . . . . . . . . . . . |
    . | . . . . . . .<2>. . . . . . . . . . . . . |
    T | . . . . . . . .<2>. . . . . . . . . . . . |
    h | . . . . . . . . .<2>. . . . . . . . . . . |
    n | . . . . . . . . . .<2>. . . . . . . . . . |
    r | . . . . . . . . . . .<2>. . . . . . . . . |
    1 | . . . . . . . . . . . .<1>. . . . . . . . |
    2 | . . . . . . . . . . . . .<1>. . . . . . . |
    3 | . . . . . . . . . . . . . .<1>. . . . . . |
    c | . . . . . . . . . . . . . . .<1>. . . . . |
    d | . . . . . . . . . . . . . . . .<1>. . . . |
    f | . . . . . . . . . . . . . . . . .<1>. . . |
    g | . . . . . . . . . . . . . . . . . .<1>. . |
    _ | . . . . . . . . . . . . . . . . . . .<.>. |
    z | . . . . . . . . . . . . . . . . . . . .<.>|
    --+-------------------------------------------+
    (row = reference; col = test)
    <BLANKLINE>

    >>> print cm.pp(sort_by_count=True, truncate=10)
      |   e a i o s t . T h |
    --+---------------------+
      |<8>. . . . . . . . . |
    e | .<6>. 3 . . . . . . |
    a | . .<4>. . . . . . . |
    i | . 1 .<1>1 . . . . . |
    o | . . . .<3>. . . . . |
    s | . . . . .<2>. . . . |
    t | . . . . . .<3>. . . |
    . | . . . . . . .<2>. . |
    T | . . . . . . . .<2>. |
    h | . . . . . . . . .<2>|
    --+---------------------+
    (row = reference; col = test)
    <BLANKLINE>

    >>> print cm.pp(sort_by_count=True, truncate=10, values_in_chart=False)
       |                   1 |
       | 1 2 3 4 5 6 7 8 9 0 |
    ---+---------------------+
     1 |<8>. . . . . . . . . |
     2 | .<6>. 3 . . . . . . |
     3 | . .<4>. . . . . . . |
     4 | . 1 .<1>1 . . . . . |
     5 | . . . .<3>. . . . . |
     6 | . . . . .<2>. . . . |
     7 | . . . . . .<3>. . . |
     8 | . . . . . . .<2>. . |
     9 | . . . . . . . .<2>. |
    10 | . . . . . . . . .<2>|
    ---+---------------------+
    (row = reference; col = test)
    Value key:
         1:  
         2: e
         3: a
         4: i
         5: o
         6: s
         7: t
         8: .
         9: T
        10: h
    <BLANKLINE>