public static function calcDepthScore($reference)
    {
        if (!self::isReferenceNumeric($reference)) {
            return null;
        }
        $reference = self::trimReference($reference);
        $sectionParts = explode('.', $reference);
        $score = 0;
        foreach ($sectionParts as $level => $part) {
            $score += $part * (1 / pow(10, $level));
        }
        return $score;
    }