�� of a graph of the model. There are two basic shapes for models with a single explanatory variable: one shape when the explanatory variable is <em>categorical</em> and another shape when the explanatory variable is quantitative.</p>
<p>We illustrate with the <code>CPS85</code> data frame. <code>CPS85</code> records a small survey of workers’ <code>wage</code>s (in 1985) and includes both numerical and categorical variables. The unit of observation is an individual worker. The categorical variable <code>