public static function likelihoodRatio(int $a, int $b, int $c, int $d) : array
{
// LL+ Positive likelihood ratio
$LL+ = $a / ($a + $c) / ($b / ($b + $d));
// LL- Negative likelihood ratio
$LL− = $c / ($a + $c) / ($d / ($b + $d));
return ['LL+' => $LL+, 'LL-' => $LL−];
}