National Health and Nutrition Examination Survey (NHANES) is a cross-sectional surveillance effort

done by the U.S. government on a sample of residents every year. NHANES makes many

measurements relevant to human health in the United States, including dietary fat intake as well as

status of many chronic diseases including CHD. If an analysis of cross-sectional data like NHANES

found that there was a strong positive association between high dietary fat intake and a CHD diagnosis

in the individuals participating, it would still be weak evidence for causation, but would be stronger

than what was found in the ecologic study presented in Figure 7-3.

Going from case series to case-control

The reason that there are two types of analytic study designs — case-control studies and cohort studies

— is that cohort study designs do not work for statistically rare conditions. We use the term

statistically rare because if someone you love gets cancer, cancer does not seem very rare. Yet, if you

enroll a cohort of thousands of individuals including your loved one (who is free of cancer) in a cohort

study and measure this cohort yearly to see who is diagnosed with cancer, it would take many years to

get enough outcomes to be able to develop the regression models (like the ones described in Chapters

16 through 23) that would be necessary for causal inference. So for statistically rare conditions like

the various cancers, you use the case-control design.

You can use a fourfold, or 2x2, table to better understand how case-control studies are

different from cohort studies. (Refer to Chapters 13 and 14 for more about 2x2 tables.) As shown

in Figure 7-4, the 2x2 table cells are labeled relative to exposure status (the rows) and outcome

or disease status (the columns). For the columns, D+ stands for having the disease (or outcome),

and D– means not having the disease or outcome. Also, for the rows, E+ means having the

exposure, and E– means not having the exposure. Cell a includes the counts of individuals in the

study who were positive for both the exposure and outcome, and cell d includes the counts of

individuals who were negative for both the exposure and outcome (a and d are concordant cells

because the exposure and outcome statuses agree). In the discordant cells, where the exposure

and outcome do not agree, is b, which represents the count of those positive for the exposure but

negative for the outcome, and c, which represents the count of those negative for the exposure but

positive for the outcome.

© John Wiley & Sons, Inc.

FIGURE 7-4: 2x2 table cells.