public static function mean(array $numbers)
{
if (empty($numbers)) {
return null;
}
return array_sum($numbers) / count($numbers);
} /**
* Sum of squares deviations
*
* ∑⟮xᵢ - μ⟯²
*
* @param array $numbers
*
* @return number
*/
public static function sumOfSquaresDeviations(array $numbers)
{
if (empty($numbers)) {
return null;
}
$μ = Average::mean($numbers);
$∑⟮xᵢ − μ⟯² = array_sum(array_map(function ($xᵢ) use($μ) {
return pow($xᵢ - $μ, 2);
}, $numbers));
return $∑⟮xᵢ − μ⟯²;
}