Changes from NSP version 0.91 to 0.93
--------------------------------------

Saiyam Kohli, kohli003@d.umn.edu
University of Minnesota, Duluth

June 11, 2006

Measures
--------

A couple of new measures have been added. These are

  1) Jaccard Coefficient

    The Jaccard Coefficient is the ratio of number of times the words
    occur together to the number of times atleast any one of the words
    occur. It is defined as:

          n11
    ---------------
    n11 + n12 + n21

    The Jaccard coefficient can also be computed by applying a
    transformation to the dice coefficient. The transformation is:

      $jaccard = $dice/(2-($dice)

    We use this computation instead of the one mentioned earlier. We have
    grouped dice and jaccard under one family.


    This measure is only available for bigrams.

  2) Poisson Stirling Measure

    The poisson stirling measure is a negative lograthimic approximation
    of the poisson-likelihood measure. It uses the stirlings firmula to
    approximate the factorial in poisson-likelihood measure. The measure
    is computed as follows.

    Posson-Stirling = n11 * ( log(n11) - log(m11) - 1)

    which is same as

    Posson-Stirling = n11 * ( log(n11/m11) - 1)

    This measure is available for bigrams as well as trigrams.


Apart from this Pointwise Mutual Information has been modified so that it
takes in a parameter named pmi_exp. This parameter is used to increase the
weight of the observed frequency count. This is useful because PMI tends to
overestimate the bigrams with lower observed frequency counts. Now the PMI
measure is computed as:

 PMI = log((n11^$exp)/m11)

The $exp is 1 by default, so the measure will compute the
Pointwise Mutual Information for the given bigram. To use a variation
of the measure, users can pass the $exp parameter using the --pmi_exp
command line option in statistic.pl. To modify the $exp from within a
program users can call the initializeStatistic method, otherwise $exp
will be taken to be 1 by default. Usage:

 $pmi->initializeStatistic(2)

The pmi measure has also been extented for trigrams.


Programs
--------

statistic.pl has been modified to take in a new command line parameter
for the PMI measure, this parameter is used to initialize the $exp
variable in PMI. If this option is not specified $exp is initialized
to 1. The option is discarded with a warning message if it is specified
for any other measure.

 The usage for statistic.pl is

 statistic.pl pmi out_pmi.stt out.cnt    - for Point Wise Mutual Information
                                           $exp is 1 in this case.

 statistic.pl --pmi_exp 2 pmi out_pmi2.stt out.cnt   - for the variant with
                                                       $exp set to 2.