treatment</code>.</p>
</dd>
<dt><code>sex</code></dt><dd><p>a factor with levels <code>female</code> and <code>male</code>.</p>
</dd>
<dt><code>age</code></dt><dd><p>the age of the patient.</p>
</dd>
<dt><code>status</code></dt><dd><p>the respiratory status (response variable), 
a factor with levels <code>poor</code> and <code>good</code>.</p>
</dd>
<dt><code>month</code></dt><dd><p>the month, each patient was examined at months 
<code>0</code>, <code>1</code>, <code>2</code>, <code>3</code> and <code>4</co