public static function cohensD($μ₁, $μ₂, $s₁, $s₂)
{
// Variance of each data set
$s₁² = $s₁ * $s₁;
$s₂² = $s₂ * $s₂;
// Pooled standard deviation
$s = sqrt(($s₁² + $s₂²) / 2);
// d
return ($μ₁ - $μ₂) / $s;
}