protected function col_discriminative_efficiency($questionstat) {
        if ($this->is_calculated_question_summary($questionstat)) {
            list($min, $max) = $questionstat->get_min_max_of('discriminativeefficiency');

            if (!is_numeric($min) && !is_numeric($max)) {
                return '';
            } else {
                return $this->format_percentage_range($min, $max, false);
            }
        } else if (!is_numeric($questionstat->discriminativeefficiency)) {
            return '';
        } else {
            return $this->format_percentage($questionstat->discriminativeefficiency);
        }
    }