public function validation($data, $files) {
        $errors = parent::validation($data, $files);

        // check the phases borders are valid
        if ($data['submissionstart'] > 0 and $data['submissionend'] > 0 and $data['submissionstart'] >= $data['submissionend']) {
            $errors['submissionend'] = get_string('submissionendbeforestart', 'mod_workshop');
        }
        if ($data['assessmentstart'] > 0 and $data['assessmentend'] > 0 and $data['assessmentstart'] >= $data['assessmentend']) {
            $errors['assessmentend'] = get_string('assessmentendbeforestart', 'mod_workshop');
        }

        // check the phases do not overlap
        if (max($data['submissionstart'], $data['submissionend']) > 0 and max($data['assessmentstart'], $data['assessmentend']) > 0) {
            $phasesubmissionend = max($data['submissionstart'], $data['submissionend']);
            $phaseassessmentstart = min($data['assessmentstart'], $data['assessmentend']);
            if ($phaseassessmentstart == 0) {
                $phaseassessmentstart = max($data['assessmentstart'], $data['assessmentend']);
            }
            if ($phasesubmissionend > 0 and $phaseassessmentstart > 0 and $phaseassessmentstart < $phasesubmissionend) {
                foreach (array('submissionend', 'submissionstart', 'assessmentstart', 'assessmentend') as $f) {
                    if ($data[$f] > 0) {
                        $errors[$f] = get_string('phasesoverlap', 'mod_workshop');
                        break;
                    }
                }
            }
        }

        // Check that the submission grade pass is a valid number.
        if (!empty($data['submissiongradepass'])) {
            $submissiongradefloat = unformat_float($data['submissiongradepass'], true);
            if ($submissiongradefloat === false) {
                $errors['submissiongradepass'] = get_string('err_numeric', 'form');
            } else {
                if ($submissiongradefloat > $data['grade']) {
                    $errors['submissiongradepass'] = get_string('gradepassgreaterthangrade', 'grades', $data['grade']);
                }
            }
        }

        // Check that the grade pass is a valid number.
        if (!empty($data['gradinggradepass'])) {
            $gradepassfloat = unformat_float($data['gradinggradepass'], true);
            if ($gradepassfloat === false) {
                $errors['gradinggradepass'] = get_string('err_numeric', 'form');
            } else {
                if ($gradepassfloat > $data['gradinggrade']) {
                    $errors['gradinggradepass'] = get_string('gradepassgreaterthangrade', 'grades', $data['gradinggrade']);
                }
            }
        }

        if (!$data['submissiontypetextavailable'] && !$data['submissiontypefileavailable']) {
            // One submission type must be available.
            $errors['submissiontypes'] = get_string('nosubmissiontype', 'workshop');
        }

        return $errors;
    }