private function compareQualityFactor(PreferenceInterface $lValue, PreferenceInterface $rValue)
    {
        if ($rValue->getQualityFactor() < $lValue->getQualityFactor()) {
            return -1;
        } elseif ($rValue->getQualityFactor() > $lValue->getQualityFactor()) {
            return 1;
        } else {
            return 0;
        }
    }