ot vary across levels of <code>am</code> or <code>hp</code>. The minimum and Q1 value of <code>hp</code> are also the same, so the marginal effects of <code>am</code> are the same in the first two results. As you can see, however, the marginal effect of <code>hp</code> differs when <code>am == 0</code> versus <code>am == 1</code> (first and second rows) and the marginal effect of <code>am</code> differs across levels of <code>hp</code> (e.g., between the first and third rows). As should be clear, the <code>