private function getDistinctReferrersMetrics($date = false)
    {
        $propertyToAccessorMapping = array(
            'numberDistinctSearchEngines'  => 'getNumberOfDistinctSearchEngines',
            'numberDistinctSocialNetworks' => 'getNumberOfDistinctSocialNetworks',
            'numberDistinctKeywords'       => 'getNumberOfDistinctKeywords',
            'numberDistinctWebsites'       => 'getNumberOfDistinctWebsites',
            'numberDistinctWebsitesUrls'   => 'getNumberOfDistinctWebsitesUrls',
            'numberDistinctCampaigns'      => 'getNumberOfDistinctCampaigns',
        );

        $result = array();
        foreach ($propertyToAccessorMapping as $property => $method) {
            $result[$property] = $this->getNumericValue('Referrers.' . $method, $date);
        }
        return $result;
    }