t</code>, however, is only: 1.513, 5.424. We can see these results in the analogous graph of the marginal effects of horsepower (below). The “front” and “back” edges of the graph are now flat (reflecting how the marginal effect of horsepower is constant across levels of horsepower), while the “front-left” and “right-back” edges of the surface are lines with slope 0.03, reflecting the coefficient on the interaction term.</p>
<div class="sourceCode"><pre class="sourceCode r"><code class="sourc