Calculate Standard Deviation (Sample)

standardDeviationSample.php

<?

     /**
     * @param array $a
     * @param bool $sample
     * @return false|float
     */
    public function calculateStandardDeviationSample(array $a, $sample = true)
    {
        $n = count($a);
        if ($n === 0) {
            trigger_error("The array has zero elements", E_USER_WARNING);

            return false;
        }
        if ($sample && $n === 1) {
            trigger_error("The array has only 1 element", E_USER_WARNING);

            return false;
        }
        $mean = array_sum($a) / $n;
        $carry = 0.0;

        foreach ($a as $val) {
            $d = ((double)$val) - $mean;
            $carry += $d * $d;
        }

        if ($sample) {
            --$n;
        }

        return sqrt($carry / $n);
    }

?>