ABSTRACT
The manatee is a slow-moving, aquatic mammal that lives in warm, shallow waters. Manatees have no natural predators, but they do share their waters with motor boats. And motor boats have propellers. Interpreting interaction estimates is tricky. It is trickier than interpreting ordinary estimates. In a simple linear regression, with no interactions, each coefficient says how much the average outcome changes when the predictor changes by one unit. And since all of the parameters have independent influences on the outcome, there is no trouble in interpreting each parameter separately. Common sorts of multilevel models are essentially massive interaction models, in which estimates (intercepts and slopes) are conditional on clusters (person, genus, village, city, galaxy) in the data. Multilevel interaction effects are complex. Models that allow for complex interactions are easy to fit to data. But they can be considerably harder to understand.
