Add option to mark a question as doubtful while answering

Can this feature be used to something? As far as i can see the user must remember which questions they thought were doubtful anyway, case the "jumper-feature" does not display doubtful answers?

Agreed. Mark doubtful seems very specific to a particular use case. What if it was more generic, something like "Bookmark," "Bookmark for later" or "I want to return to this question for further consideration?"

And then couple this with a view of the marked questions available from a utility navigation item.

I found a solution for my self, by using the theme function for the jumper, i think it could have been done better, but it works.

function mytheme_quiz_jumper($vars) {
  $quiz = array();

  foreach ($_SESSION as $key => $value) {
    if (is_array($value) && isset($value['result_id'])) {
      $quiz = $value;
    }
  }
  if (empty($quiz)) {
    $current = $vars['current'];
    $num_questions = $vars['num_questions'];
    $output = '<select name="quiz-jumper" class="form-select" id="quiz-jumper">';
    for ($i = 1; $i <= $num_questions; $i++) {
      $extra = $i == $current ? ' selected="selected"' : '';
      $output .= '<option value="' . $i . '"' . $extra . '>' . $i . '</option>';
    }
    $output .= '</select><span id="quiz-jumper-no-js">' . $current . '</span>';
  }
  else {
    //dpm($vars);
    //dpm($quiz);
    $all = array_merge($quiz['previous_quiz_questions'], $quiz['quiz_questions']);
    //dpm($all);

    $current = $vars['current'];
    $y = 1;
    $output = '<select name="quiz-jumper" class="form-select" id="quiz-jumper">';

    foreach ($all as $question) {
      $extra = $y == $current ? ' selected="selected"' : '';
      if (mytheme_check_if_doubt($quiz['result_id'], $question['nid'], $question['vid'])) {
        $output .= '<option value="' . $y . '"' . $extra . '>' . $y . ' - In doubt</option>';
      }
      else {
        $output .= '<option value="' . $y . '"' . $extra . '>' . $y . '</option>';
      }
      $y++;
    }
    $output .= '</select><span id="quiz-jumper-no-js">' . $current . '</span>';
  }
  drupal_add_js('
    (function ($) {
      Drupal.behaviors.quizJumper = {
        attach: function(context, settings) {
          $("#quiz-jumper:not(.quizJumper-processed)", context).show().addClass("quizJumper-processed").change(function(){
            $("[name=jump_to_question]").val($(this).val());
            $("#edit-submit").trigger("click");
          });
          $("#quiz-jumper-no-js:not(.quizJumper-processed)").hide().addClass("quizJumper-processed");
        }
      };
    })(jQuery);
  ', array('type' => 'inline', 'scope' => JS_DEFAULT));
  return $output;
}

function mytheme_check_if_doubt($res_id, $quiz_nid, $quiz_vid) {
  $result = db_select('quiz_node_results_answers', 'q')
    ->fields('q')
    ->condition('result_id', $res_id,'=')
    ->condition('question_nid', $quiz_nid,'=')
    ->condition('question_vid', $quiz_vid,'=')
    ->execute()
    ->fetchAssoc();

  if (isset($result) && $result['is_doubtful'] == 1) {
    return TRUE;
  }
  else {
    return FALSE;
  }
}

I feel this is a little hacky, but it seems to work.

andeersg & ezraw.

The doubtful mark is usefull when the concepts covered by question need to be reinforced.

The student answer the question but "not sure" about "why this answer". He/she needs to review the theory/concetps behind the question.

Once quiz has been completed, the student/tutor can review all doubtful question to reinforce concepts.