=head1 NAME

Statistics::Basic::Covariance - find the covariance between two lists

=head1 SYNOPSIS

Invoke it this way:

    my $covariance = covariance( [1,2,3], [1,2,3] );

Or this way:

    my $v1  = vector(1,2,3);
    my $v2  = vector(1,2,3);
    my $cov = cov($v1,$v2);

And then either query the values or print them like so:

    print "The covariance between $v1 and $v2: $covariance\n";
    my $cq = $cov->query;
    my $c0 = 0+$cov;

Create a 20 point "moving" covariance like so:

    use Statistics::Basic qw(:all nofill);

    my $sth = $dbh->prepare("select col1,col2 from data where something");
    my $len = 20;
    my $cov = cov();
       $cov->set_size($len);

    $sth->execute or die $dbh->errstr;
    $sth->bind_columns( my ($lhs, $rhs) ) or die $dbh->errstr;

    my $count = $len;
    while( $sth->fetch ) {
        $cov->insert( $lhs, $rhs );
        if( defined( my $c = $cov->query ) ) {
            print "Covariance: $c\n";
        }

        # This would also work:
        # print "Covariance: $cov\n" if $cov->query_filled;
    }

=head1 METHODS

This list of methods skips the methods inherited from
L<Statistics::Basic::_TwoVectorBase>
(things like L<query()|Statistics::Basic::_TwoVectorBase/query()>,
L<insert()|Statistics::Basic::_TwoVectorBase/insert()>,
and L<ginsert()|Statistics::Basic::_TwoVectorBase/ginsert()>).

=over 4

=item B<new()>

Create a new L<Statistics::Basic::Covariance> object.  This function takes two
arguments -- which can either be arrayrefs or L<Statistics::Basic::Vector>
objects.  This function is called when the
L<covariance()|Statistics::Basic/covariance() cov()> shortcut-function
is called.

=item B<query_vector1()>

Return the L<Statistics::Basic::Vector> for the first vector.

=item B<query_vector2()>

Return the L<Statistics::Basic::Vector> object for the second vector.

=item B<query_mean1()>

Returns the L<Statistics::Basic::Mean> object for the first vector.

=item B<query_mean2()>

Returns the L<Statistics::Basic::Mean> object for the second vector.

=back

=head1 OVERLOADS

This object is overloaded.  It tries to return an appropriate string for the
calculation or the value of the computation in numeric context.

In boolean context, this object is always true (even when empty).

=head1 AUTHOR

Paul Miller C<< <jettero@cpan.org> >>

=head1 COPYRIGHT

Copyright 2009 Paul Miller -- Licensed under the LGPL

=head1 SEE ALSO

perl(1), L<Statistics::Basic>, L<Statistics::Basic::_TwoVectorBase>, L<Statistics::Basic::Vector>


=cut