s="co">#&gt; Missing values: -92, -91</span></span></code></pre></div>
<p>All types of variables (categorical and numerical) can have declared
missing values. There are situations when values are not missing
randomly but with a specific reason. In social research, respondents
often can not or do not want to provide an answer for a certain
question, be it categories of opinions or pure numerical answers like
age, income, etc.</p>
<p>Base R has a clear distinction between numerical and categorical
variables. 