Sometimes studies with ecologic measures of exposure or outcome report rate differences (called "absolute rate differences") and rate ratios (called "incident rate ratios"). How are researchers able to estimate these values in an ecologic study? Recall that most cross-sectional studies measure the prevalence of a specific outcome, and thus, common measures of association for cross-sectional studies include prevalence difference, prevalence ratio, and prevalence odds ratio.

Hint: Think back to definitions of risks and rates compared to prevalence.What would be different about a risk or rate outcome? What is it about that outcome that is so unique and/or how many times can you "get" the outcome?