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

        $returnValue .= '<setOutcomeValue identifier="'.$this->outcomeIdentifier.'"><sum>';
        foreach ($this->components as $irp) {
        	$returnValue .= '<variable identifier="'.$irp->getOutcome()->getIdentifier().'" />';
        }
        $returnValue .= '</sum></setOutcomeValue>';

        return (string) $returnValue;
    }