protected function col_effective_weight($questionstat) {
        global $OUTPUT;

        if ($this->is_calculated_question_summary($questionstat)) {
            list($min, $max) = $questionstat->get_min_max_of('effectiveweight');

            if (is_null($min) && is_null($max)) {
                return '';
            } else {
                list( , $negcovar) = $questionstat->get_min_max_of('negcovar');
                if ($negcovar) {
                    $min = get_string('negcovar', 'quiz_statistics');
                }

                return $this->format_range($min, $max);
            }
        } else if (is_null($questionstat->effectiveweight)) {
            return '';
        } else if ($questionstat->negcovar) {
            $negcovar = get_string('negcovar', 'quiz_statistics');

            if (!$this->is_downloading()) {
                $negcovar = html_writer::tag('div',
                        $negcovar . $OUTPUT->help_icon('negcovar', 'quiz_statistics'),
                        array('class' => 'negcovar'));
            }

            return $negcovar;
        } else {
            return $this->format_percentage($questionstat->effectiveweight, false);
        }
    }