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
- Good for studying rare conditions or diseases
- Less time needed to conduct the study because the condition or disease has already occurred
- Lets you simultaneously look at multiple risk factors
- Useful as initial studies to establish an association
- Can answer questions that could not be answered through other study designs
Disadvantages
- Retrospective studies have more problems with data quality because they rely on memory and people with a condition will be more motivated to recall risk factors (also called recall bias).
- Not good for evaluating diagnostic tests because it’s already clear that the cases have the condition and the controls do not
- It can be difficult to find a suitable control group
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 to investigate zinc oxide as a more effective skin cancer prevention measure would involve comparing a group of former lifeguards who regularly wore zinc oxide with another group of former lifeguards who wore absorbent sunscreens to compare how often each group later developed cancer on their cheeks or noses.
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
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.
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.
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.