class Covariance {
  
        method Covariance($xpop,$ypop) {
                my $ex = $xpop.Expectance();
                my $ey = $ypop.Expectance();

                my $cov = 0.0;

                for $xpop.population, $ypop.population -> $p,$q {
                        $cov += ($p - $ex)  * ($q - $ey);
                }

                return $cov / $xpop.population.length;

        }
}