public static function cohensQ($r₁, $r₂)
{
if ($r₁ >= 1 || $r₂ >= 1) {
throw new Exception\OutOfBoundsException('r must be greater than or equal to 1');
}
$½ = 0.5;
return abs($½ * log((1 + $r₁) / (1 - $r₁)) - $½ * log((1 + $r₂) / (1 - $r₂)));
}