Caption: The Counterfactual Ideal
Text: The ideal comparison group consists of exactly the same individuals in the exposed group had they not been exposed. The advantage, of course, is that any difference in risk between the groups could be attributed to the exposure of interest, without any distortion from other (confounding) factors that might influence the outcome. Since it is impossible for the same person to be exposed and unexposed simultaneously, epidemiologists must select different sets of people who are as similar as possible.