 Statistics::Basic::Covariance(3) find the covariance between two lists

## 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;
}
```

## METHODS

This list of methods skips the methods inherited from Statistics::Basic::_TwoVectorBase (things like query(), insert(), and ginsert()).
new()
Create a new Statistics::Basic::Covariance object. This function takes two arguments --- which can either be arrayrefs or Statistics::Basic::Vector objects. This function is called when the covariance() shortcut-function is called.
query_vector1()
Return the Statistics::Basic::Vector for the first vector.
query_vector2()
Return the Statistics::Basic::Vector object for the second vector.
query_mean1()
Returns the Statistics::Basic::Mean object for the first vector.
query_mean2()
Returns the Statistics::Basic::Mean object for the second vector.