<p>
  The questionnaire system uses specific codes to represent different types of missing responses:
</p>

<p>
  <b>-99 (Skipped):</b> When a participant encounters a question but chooses not to answer it
  (e.g., leaves it blank), the system records "-99".
</p>

<p>
  <b>-77 (Filtered Out):</b> When a question is not displayed to a participant due to filter conditions,
  the system records "-77".
</p>

<p><b>Setting Up Filter Conditions:</b></p>

<ul>
  <li>To hide a question when participants skip another question: <code>hide if [question] == -99</code></li>
  <li>To hide a question when another question was filtered out: <code>hide if [question] == -77</code></li>
  <li>
    To hide a question for any type of missing response: <code>hide if [question] == -99 OR [question] == -77</code>.
    If you only use positive integers as response values, you may also be able to simplify the condition as <code>hide if [question] < 0</code>
  </li>
</ul>
