Ted Pedersen

NAME

WordNet::Similarity::path - Perl module for computing semantic relatedness of word senses by counting nodes in the noun and verb WordNet 'is-a' hierarchies.

SYNOPSIS

  use WordNet::Similarity::path;

  use WordNet::QueryData;

  my $wn = WordNet::QueryData->new();

  my $path = WordNet::Similarity::path->new($wn);

  my $value = $measure->getRelatedness("car#n#1", "bus#n#2");

  my ($error, $errorString) = $measure->getError();

  die "$errorString\n" if($error);

  print "car (sense 1) <-> bus (sense 2) = $value\n";

DESCRIPTION

This module computes the semantic relatedness of word senses by counting the number of nodes along the shortest path between the senses in the 'is-a' hierarchies of WordNet. The path lengths include the end nodes. For example, the path between shrub#n#1 and tree#n#1 is shrub#n#1 - woody_plant#n#1 - tree#n#1.

Since a longer path length indicate less relatedness, the relatedness value returned is the multiplicative inverse of the path length (distance) between the two concepts: relatedness = 1 / distance. If the two concepts are identical, then the distance between them is one; therefore, their relatedness is also 1. If no path is found, then a large negative number is returned and an error is generated (see getError()).

The following methods are defined:

$path->getRelatedness()

Computes the relatedness of two word senses using a node counting scheme. The relatedness score is inversely proportional to the number of nodes along the shortest path between the two word senses.

Parameters: two word senses in "word#pos#sense" format.

Returns: Unless a problem occurs, the return value is the relatedness score, which belongs to the interval (0, 1]. If no path exists between the two word senses, then a large negative number is returned. If an error occurs, then the error level is set to non-zero and an error string is created (see the description of getError()). Note: the error level will also be set to 1 and an error string will be created if no path exists between the words.

Discussion

If the two synsets being compared are the same, then the resulting relatedness score will be 1. For exaple, the score for car#n#1 and car#n#1 is 1.

Due to multiple inheritance in the WordNet taxonomies, it is possible for there to be a tie for the shortest path between synsets. If such a tie occurs, then all of the paths that are tied will be printed to the trace string.

The relatedness value returned by getRelatedness() is the multiplicative inverse of the path length between the two synsets (1/path_length). This has a slightly subtle effect: it shifts the relative magnitude of scores. For example, if we have the following pairs of synsets with the given path lengths:

  synset1 synset2: 3
  synset3 synset4: 4
  synset5 synset6: 5

We observe that the difference in the score for synset1-synset2 and synset3-synset4 is the same as for synset3-synset4 and synset5-synset6. When we take the multiplicative inverse of them, we get:

  synset1 synset2: .333
  synset3 synset4: .25
  synset5 synset6: .2

Now the difference between the scores for synset3-synset4 is less than the difference for synset1-synset2 and synset3-synset4. This can have negative consequences when computing correlation coefficients. It might be useful to compute relatedness as max_distance - path_length, where max_distance is the longest possible shortest path between two synsets. The original path length can be easily determined by taking the multiplicative inverse of the returned relatedness score: 1/score = 1/(1/path_length) = path_length. The computation of max_distance is left as an exercise for the reader.

If two different word senses (wps strings) are given as input to getRelatedness, but both word senses belong to the same synset, then 1 is returned (e.g., car#n#1 and auto#n#1 both belong to the same synset).

Usage

The semantic relatedness modules in this distribution are built as classes that define the following methods:

  new()
  getRelatedness()
  getError()
  getTraceString()

See the WordNet::Similarity(3) documentation for details of these methods.

Typical Usage Examples

To create an object of the path measure, we would have the following lines of code in the Perl program.

   use WordNet::Similarity::path;
   $measure = WordNet::Similarity::path->new($wn, '/home/sid/path.conf');

The reference of the initialized object is stored in the scalar variable '$measure'. '$wn' contains a WordNet::QueryData object that should have been created earlier in the program. The second parameter to the 'new' method is the path of the configuration file for the path measure. If the 'new' method is unable to create the object, '$measure' would be undefined. This, as well as any other error/warning may be tested.

   die "Unable to create object.\n" if(!defined $measure);
   ($err, $errString) = $measure->getError();
   die $errString."\n" if($err);

To find the semantic relatedness of the first sense of the noun 'car' and the second sense of the noun 'bus' using the measure, we would write the following piece of code:

   $relatedness = $measure->getRelatedness('car#n#1', 'bus#n#2');

To get traces for the above computation:

   print $measure->getTraceString();

However, traces must be enabled using configuration files. By default traces are turned off.

CONFIGURATION FILE

The behavior of the measures of semantic relatedness can be controlled by using configuration files. These configuration files specify how certain parameters are initialized within the object. A configuration file may be specified as a parameter during the creation of an object using the new method. The configuration files must follow a fixed format.

Every configuration file starts with the name of the module ON THE FIRST LINE of the file. For example, a configuration file for the path module will have on the first line 'WordNet::Similarity::path'. This is followed by the various parameters, each on a new line and having the form 'name::value'. The 'value' of a parameter is optional (in case of boolean parameters). In case 'value' is omitted, we would have just 'name::' on that line. Comments are supported in the configuration file. Anything following a '#' is ignored.

The module parses the configuration file and recognizes the following parameters:

trace

The value of this parameter specifies the level of tracing that should be employed for generating the traces. This value is an integer equal to 0, 1, or 2. If the value is omitted, then the default value, 0, is used. A value of 0 switches tracing off. A value of 1 or 2 switches tracing on. A trace level of 1 means the synsets are represented as word#pos#sense strings, while for level 2, the synsets are represented as word#pos#offset strings.

cache

The value of this parameter specifies whether or not caching of the relatedness values should be performed. This value is an integer equal to 0 or 1. If the value is omitted, then the default value, 1, is used. A value of 0 switches caching 'off', and a value of 1 switches caching 'on'.

maxCacheSize

The value of this parameter indicates the size of the cache, used for storing the computed relatedness value. The specified value must be a non-negative integer. If the value is omitted, then the default value, 5,000, is used. Setting maxCacheSize to zero has the same effect as setting cache to zero, but setting cache to zero is likely to be more efficient. Caching and tracing at the same time can result in excessive memory usage because the trace strings are also cached. If you intend to perform a large number of relatedness queries, then you might want to turn tracing off.

rootNode

The value of this parameter indicates whether or not a unique root node should be used. In WordNet, there is no unique root node for the noun and verb taxonomies. If this parameter is set to 1 (or if the value is omitted), then certain measures (wup, path, lch, res, lin, and jcn) will "fake" a unique root node. If the value is set to 0, then no unique root node will be used. If the value is omitted, then the default value, 1, is used.

SEE ALSO

perl(1), WordNet::Similarity(3), WordNet::QueryData(3)

http://www.cs.utah.edu/~sidd

http://wordnet.princeton.edu

http://www.ai.mit.edu/~jrennie/WordNet

http://groups.yahoo.com/group/wn-similarity

AUTHORS

 Ted Pedersen, University of Minnesota Duluth
 tpederse at d.umn.edu

 Siddharth Patwardhan, University of Utah, Salt Lake City
 sidd at cs.utah.edu

 Jason Michelizzi, University of Minnesota Duluth
 mich0212 at d.umn.edu

BUGS

None.

To report bugs, go to http://groups.yahoo.com/group/wn-similarity.

COPYRIGHT AND LICENSE

Copyright (c) 2005, Ted Pedersen, Siddharth Patwardhan and Jason Michelizzi

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to

    The Free Software Foundation, Inc.,
    59 Temple Place - Suite 330,
    Boston, MA  02111-1307, USA.

Note: a copy of the GNU General Public License is available on the web at http://www.gnu.org/licenses/gpl.txt and is included in this distribution as GPL.txt.




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