elpful because it allows for calculation of marginal effects for <em>counterfactual</em> datasets (e.g., what if all women were instead men? what if all democracies were instead autocracies? what if all foreign cars were instead domestic?).</p>
<p>As an example, if we wanted to know if the marginal effect of horsepower (<code>hp</code>) on fuel economy differed across different types of automobile transmissions, we could simply use <code>at</code> to obtain separate marginal effect estimates for our data as