Study Design 101

Case Control Study

Definition

A study that compares patients who have a disease or outcome of interest (cases) with patients who do not have the disease or outcome (controls), and looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease.

Case control studies are observational because no intervention is attempted and no attempt is made to alter the course of the disease. The goal is to retrospectively determine the exposure to the risk factor of interest from each of the two groups of individuals: cases and controls. These studies are designed to estimate odds.

Case control studies are also known as "retrospective studies" and "case-referent studies."

Advantages

Disadvantages

Design pitfalls to look out for

Care should be taken to avoid confounding, which arises when an exposure and an outcome are both strongly associated with a third variable. Controls should be subjects who might have been cases in the study but are selected independent of the exposure. Cases and controls should also not be "over-matched."

Is the control group appropriate for the population? Does the study use matching or pairing appropriately to avoid the effects of a confounding variable? Does it use appropriate inclusion and exclusion criteria?

Fictitious Example

There is a suspicion that zinc oxide, the white non-absorbent sunscreen traditionally worn by lifeguards is more effective at preventing sunburns that lead to skin cancer than absorbent sunscreen lotions. A case-control study was conducted to investigate if exposure to zinc oxide is a more effective skin cancer prevention measure. The study involved comparing a group of former lifeguards that had developed cancer on their cheeks and noses (cases) to a group of lifeguards without this type of cancer (controls) and assess their prior exposure to zinc oxide or absorbent sunscreen lotions.

This study would be retrospective in that the former lifeguards would be asked to recall which type of sunscreen they used on their face and approximately how often. This could be either a matched or unmatched study, but efforts would need to be made to ensure that the former lifeguards are of the same average age, and lifeguarded for a similar number of seasons and amount of time per season.

Real-life Examples

Chambers, C. D., Hernandez-Diaz, S., Van Marter, L. J., Werler, M. M., Louik, C., & Jones, K. L. et al. (2006). Selective serotonin-reuptake inhibitors and risk of persistent pulmonary hypertension of the newborn. New England Journal of Medicine, 354(6), 579-587.

This study used a matched design, matching infants who had persistent pulmonary hypertension with infants who did not have it, and compared the rates of exposure to SSRIs.

Smedby, K. E., Hjalgrim, H., Askling, J., Chang, E. T., Gregersen, H., & Porwit-MacDonald, A. et al. (2006). Autoimmune and chronic inflammatory disorders and risk of non-hodgkin lymphoma by subtype. Journal of the National Cancer Institute, 98(1), 51-60.

This study matched patients with non-Hodgkin lymphoma (NHL) with control subjects and compared their history of autoimmune and chronic inflammatory disorders, markers of severity, and treatment. It found that the risk of NHL was increased in association with rheumatoid arthritis, primary Sjögren syndrome, systemic lupus erythematosus, and celiac disease.

Teo, K. K., Ounpuu, S., Hawken, S., Pandey, M., Valentin, V., & Hunt, D. et al. (2006). Tobacco use and risk of myocardial infarction in 52 countries in the INTERHEART study: A case-control study. Lancet, 368(9536), 647-658.

This study looked at the relation between risk of acute myocardial infarction and current or former smoking, type of tobacco, amount smoked, effect of smokeless tobacco, and exposure to secondhand smoke.

Related Formulas

Related Terms

Now test yourself!

1. Case Control Studies are prospective in that they follow the cases and controls over time and observe what occurs.

a) True
b) False

2. Which of the following is an advantage of Case Control Studies?

a) They can simultaneously look at multiple risk factors.
b) They are useful to initially establish an association between a risk factor and a disease or outcome.
c) They take less time to complete because the condition or disease has already occurred.
d) b and c only
e) a, b, and c

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