Caption: "Free of Disease"
Text: Note that in this context the term "free of disease" really means that the subjects do not yet have the outcome that we are interested in studying. Furthermore, "free of disease" does not mean that the subjects are healthy. For example, one might identify a cohort of subjects who had had a heart attack and group them into sub-cohorts that smoked or didn't smoke, and compare their mortality rates. The outcome of interest is death, not heart disease, and the subjects in this hypothetical study were clearly "at risk" of developing the outcome of interest. Moreover, they weren't really healthy, because they all had heart disease.