For example, in a study presented in Chapter 16 of Epidemiology For Dummies by Amal K. Mitra

(Wiley), the experimental unit is a country, and 15 countries were included in the analysis. The

exposure being investigated is fat intake from diet (which was operationalized as average saturated fat

intake as a percentage of energy in the diet). The outcome was deaths from coronary heart disease

(CHD), operationalized as 50-year CHD deaths per 1,000 person-years (see Chapter 15 for more

about rates in person-years). Figure 7-3 presents the results in the form of a scatter plot.

© John Wiley & Sons, Inc.

FIGURE 7-3: Ecologic study results.

As shown in Figure 7-3, the country’s average value of the outcome (rate of CHD deaths) is plotted on

the y-axis because it’s the outcome. The exposure, average dietary fat intake for the country, is plotted

on the x-axis. The 15 countries in the study are plotted according to their x-y coordinates. Notice that

the United States is in the upper-right quadrant of the scatter plot because it has high rates of both the

exposure and outcome. The strong, positive value of correlation coefficient r (which is 0.92) indicates

that there is a strong positive bivariate association between the exposure and outcome, which is weak

evidence for causality (flip to Chapter 15 for more on correlation).

But the problem with ecologic studies is that the experimental unit is a whole population —

not an individual. What if the individuals in the United States who ate low-fat diets were actually

the ones to die of CHD? And what if the ones who ate high-fat diets were more likely to die of

something else? Attributing the behavior of a group to an individual is called the ecologic

fallacy, and can be a problem with interpreting results like the ones shown in Figure 7-3.

That is why we also have cross-sectional studies, where the experimental unit is an individual, not a

population. A cross-sectional study takes measurements of individuals at one point in time — either

through an in-person hands-on examination, or by survey (over the phone, Internet, or in person). The