ve. This is clear looking at the “front” or “back” edges of the surface, which are straight-linear increases. The slope of those edges is 0.03 (the coefficient on the interaction term).</p>
<p>If we then take the partial derivative with respect to <code>hp</code> (to obtain the marginal effect of horsepower), the equation is: <code>d_mpg/d_hp = (-0.12) + (0.03 * wt)</code>. When <code>wt</code> is zero, this partial derivative (or marginal effect) is -0.12 miles/gallon. The observed range of <code>w