ack using <code>summary()</code>, but it’s an extremely long output (one row for each estimate times the number of unique combinations of <code>at</code> values, so 120 rows in this instance).</p>
<p>An easier way to understand all of this is to use graphing. The <code><a href="../reference/cplot.html">cplot()</a></code> function, in particular, is useful here. We can, for example, plot the marginal effect of axle ratio across levels of vehicle weight, separately for automatic vehicles (in red) and manual