Bio::SearchDist(3) A perl wrapper around Sean Eddy's histogram object


$dis = Bio::SearchDist->new();
foreach $score ( @scores ) {
if( $dis->fit_evd() ) {
foreach $score ( @scores ) {
$evalue = $dis->evalue($score);
print "Score $score had an evalue of $evalue\n";
} else {
warn("Could not fit histogram to an EVD!");


The Bio::SearchDist object is a wrapper around Sean Eddy's excellent histogram object. The histogram object can bascially take in a number of scores which are sensibly distributed somewhere around 0 that come from a supposed Extreme Value Distribution. Having add all the scores from a database search via the add_score method you can then fit a extreme value distribution using fit_evd(). Once fitted you can then get out the evalue for each score (or a new score) using evalue($score).

The fitting procedure is better described in Sean Eddy's own code (available from, or in the histogram.h header file in Compile/SW). Bascially it fits a EVD via a maximum likelhood method with pruning of the top end of the distribution so that real positives are discarded in the fitting procedure. This comes from an orginally idea of Richard Mott's and the likelhood fitting is from a book by Lawless [should ref here].

The object relies on the fact that the scores are sensibly distributed around about 0 and that integer bins are sensible for the histogram. Scores based on bits are often ideal for this (bits based scoring mechanisms is what this histogram object was originally designed for).


The original code this was based on comes from the histogram module as part of the HMMer2 package. Look at

Its use in Bioperl is via the Compiled XS extension which is cared for by Ewan Birney ([email protected]). Please contact Ewan first about the use of this module


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The rest of the documentation details each of the object methods. Internal methods are usually preceded with a _


 Title   : add_score
 Usage   : $dis->add_score(300);
 Function: Adds a single score to the distribution
 Returns : nothing
 Args    :


 Title   : fit_evd
 Usage   : $dis->fit_evd();
 Function: fits an evd to the current distribution
 Returns : 1 if it fits successfully, 0 if not
 Args    :


 Title   : fit_Gaussian
 Usage   :
 Example :
 Returns :
 Args    :


 Title   : evalue
 Usage   : $eval = $dis->evalue($score)
 Function: Returns the evalue of this score
 Returns : float
 Args    :


 Title   : _engine
 Usage   : $obj->_engine($newval)
 Function: underlyine bp_sw:: histogram engine
 Returns : value of _engine
 Args    : newvalue (optional)