public function validate() {
        global $DB;
        $params = array_keys($this->params);
        $method = ($this->method == BADGE_CRITERIA_AGGREGATION_ALL);
        $singleparam = (count($params) == 1);

        foreach ($params as $param) {
            // Perform check if there only one parameter with any type of aggregation,
            // Or there are more than one parameter with aggregation ALL.
            if (($singleparam || $method) && !$DB->record_exists('cohort', array('id' => $param))) {
                return array(false, get_string('error:invalidparamcohort', 'badges'));
            }
        }

        return array(true, '');
    }