public function prune(array $results)
    {
        $mean = array_sum($results) / count($results);
        $deviation = $this->deviations * $this->standardDeviation($results);
        $lower = $mean - $deviation;
        $upper = $mean + $deviation;

        return array_values(array_filter($results, function ($val) use ($lower, $upper) {
                return $val >= $lower && $val <= $upper;
            }));
    }