public function toQTI()
    {
        $returnValue = (string) '';

        
		$returnValue = '<responseCondition>
		    <responseIf>
	            <isNull>
	                <variable identifier="'.$this->getResponse()->getIdentifier().'" />
	            </isNull>
		        <setOutcomeValue identifier="'.$this->getOutcome()->getIdentifier().'">
		        	<baseValue baseType="'.$this->getOutcome()->getAttributeValue('baseType').'">0</baseValue>
		        </setOutcomeValue>
		    </responseIf>
		</responseCondition>';
        

        return (string) $returnValue;
    }