public function getForcedVariation($experimentKey, $userId)
    {
        if (!$this->validateInputs(
            [
                self::EXPERIMENT_KEY =>$experimentKey,
                self::USER_ID => $userId
            ]
        )) {
            return null;
        }

        $forcedVariation = $this->_config->getForcedVariation($experimentKey, $userId);
        if (isset($forcedVariation)) {
            return $forcedVariation->getKey();
        } else {
            return null;
        }
    }