292 PART 5 Looking for Relationships with Correlation and Regression

In this chapter, we revisit the concept of confounding from Chapter 7 and explain

how to choose candidate covariates for your regression model. We also discuss

modeling approaches and explain how to add interaction terms to your

final model.

Staying Clearheaded about Confounding

Chapter 7 discusses study design and terminology in epidemiology. As a reminder,

in epidemiology, exposure refers to a factor you hypothesize to cause a disease (or

outcome). In your regression model, the outcome is the dependent variable. The

exposure will be one of the covariates in your model. But what other covariates

belong in the model? How do you decide on a collection of candidate-independent

variables that you would even consider putting in a model with the exposure? The

answer is that you choose them on the basis of their status as a potential

confounder.

A confounder is a factor that meets these three criteria:»

» ^{It is associated with the exposure.»}

» ^{It is associated with the outcome.»}

» ^{It is not on the causal pathway between the exposure and outcome.}

As an example, look at Figure  20-1, which illustrates a study of patients with

Type II diabetes where there is a hypothesized causal relationship between the

exposure of having served in the military and the negative outcome of having an

amputation due to diabetic complications.

As shown Figure 20-1, inability to exercise and low income are both seen as potential

confounders. That is because they are associated with both the exposure of mili-

tary service and the outcome of amputation, and they are not on the causal path-

way between military service and amputation. In other words, what is causing the

outcome of amputation is not also causing the patient’s inability to exercise, nor

is it also causing the patient to have low income. But whatever is causing the

patient’s amputation is also causing the patient’s retinopathy. That’s because

Type II diabetes causes poor circulation, which causes both retinopathy and

amputation. This means that retinopathy and amputation are on the same causal

pathway, and retinopathy cannot be considered a potential confounder.