public function process()
    {
        $testSession = $this->getState();
        $itemSubset = $this->getItemSubset();
        $numberResponded = 0;

        foreach ($itemSubset as $item) {
            $itemSessions = $testSession->getAssessmentItemSessions($item->getIdentifier());
            
            if ($itemSessions !== false) {
                foreach ($itemSessions as $itemSession) {
                    if ($itemSession->isResponded() === true) {
                        $numberResponded++;
                    }
                }    
            }
        }

        return new QtiInteger($numberResponded);
    }