function getQuantity(){
		if($this->task_activity_id > 0){
			$ta = new SI_TaskActivity();
			if($ta->get($this->task_activity_id) === FALSE){
				$this->error = 'SI_InvoiceLineLink::getQuantity(): Error getting task activity: '.$ta->getLastError();
				return FALSE;	
			}
			
			return $ta->getQuantity();
		}elseif($this->expense_id > 0){
			return 1.0;
		}elseif($this->payment_schedule_id > 0){
			return 1.0;
		}
		
		return 'Unknown';
	}