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WordNet::Similarity::lUser)Contributed Perl DocumenWordNet::Similarity::lch(3)

       WordNet::Similarity::lch	- Perl module for computing semantic
       relatedness of word senses using	the method described by	Leacock	and
       Chodorow	(1998).

	 use WordNet::Similarity::lch;

	 use WordNet::QueryData;

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

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

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

	 ($error, $errorString)	= $myobj->getError();

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

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

       This module computes the	semantic relatedness of	word senses according
       to a method described by	Leacock	and Chodorow (1998). This method
       counts up the number of edges between the senses	in the 'is-a'
       hierarchy of WordNet.  The value	is then	scaled by the maximum depth of
       the WordNet 'is-a' hierarchy. A relatedness value is obtained by	taking
       the negative log	of this	scaled value.

	   This	method is internally called to determine the parts of speech
	   this	measure	is capable of dealing with.

	   Parameters: none.

	   Returns: none.

       $lch->getRelatedness ($synset1, $synset2)
	   Computes the	relatedness of two word	senses using a node counting
	   scheme.  For	details	on how relatedness is computed,	see the
	   Discussion section below.

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

	   Returns: Unless a problem occurs, the return	value is the
	   relatedness score.  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.

       The relatedness measure proposed	by Leacock and Chodorow	is
       -logA (lengthA /A (2A *A	D)), where length is the length	of the
       shortest	path between the two synsets (using node-counting) and D is
       the maximum depth of the	taxonomy.

       The fact	that the lch measure takes into	account	the depth of the
       taxonomy	in which the synsets are found means that the behavior of the
       measure is profoundly affected by the presence or absence of a unique
       root node.  If there is a unique	root node, then	there are only two
       taxonomies: one for nouns and one for verbs.  All nouns,	then, will be
       in the same taxonomy and	all verbs will be in the same taxonomy.	 D for
       the noun	taxonomy will be somewhere around 18, depending	upon the
       version of WordNet, and for verbs, it will be 14. If the	root node is
       not being used, however,	then there are nine different noun taxonomies
       and over	560 different verb taxonomies, each with a different value for

       If the root node	is not being used, then	it is possible for synsets to
       belong to more than one taxonomy.  For example, the synset containing
       turtledove#n#2 belongs to two taxonomies:  one rooted at	group#n#1 and
       one rooted at entity#n#1.  In such a case, the relatedness is computed
       by finding the LCS that results in the shortest path between the
       synsets.	 The value of D, then, is the maximum depth of the taxonomy in
       which the LCS is	found.	If the LCS belongs to more than	one taxonomy,
       then the	taxonomy with the greatest maximum depth is selected (i.e.,
       the largest value for D).

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


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

       Typical Usage Examples

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

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

	  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.

       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

       Every configuration file	starts with the	name of	the module ON THE
       FIRST LINE of the file. For example, a configuration file for the
       WordNet::Similarity::lch	module will have on the	first line
       'WordNet::Similarity::lch'. 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 till the	end of the line.

       The module parses the configuration file	and recognizes the following

	   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.

	   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'.

	   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.

	   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.

	   The value for this parameter	should be a string that	specifies the
	   location of a taxonomy depths file (as generated by
	   If no path is specified, then the default file is used, which was
	   generated when the Similarity package was installed.

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

	Ted Pedersen, University of Minnesota Duluth
	tpederse at

	Siddharth Patwardhan, University of Utah, Salt Lake City
	sidd at

	Jason Michelizzi, University of	Minnesota Duluth
	mich0212 at


       To report bugs, go to or
       e-mail tpederse at

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

       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
       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 <>	and is included	in this
       distribution as GPL.txt.

perl v5.24.1			  2008-03-27	   WordNet::Similarity::lch(3)


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