nterpretation become almost immediately clear. In this first model, we’ll use the <code>mtcars</code> dataset to understand vehicle fuel economy as a function of <code>drat</code> (rear axle ratio), <code>wt</code> (weight), and their interaction. As Brambor et al. (2006) make clear, the most common mistake in such models is estimating the model without the constituent variables. We can see why this is a problem by estimating the model with and without the constituent terms:</p>
<div class="sourceCode"><p