Cases are identified first, then controls.
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 controlsand 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: These studies are designed to estimate odds.
Case control studies are also known as "retrospective studies" and "case-referent studies. 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. 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. Selective serotonin-reuptake inhibitors and risk of persistent pulmonary hypertension of the newborn.
New England Journal of Medicine, 6 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. Autoimmune and chronic inflammatory disorders and risk of non-hodgkin lymphoma by subtype.
Journal of the National Cancer Institute, 98 1 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.
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.Available information indicates that roughly 30% of the controls are not vaccinated, and we wish to have an 80% chance of detecting whether the odds ratio is significantly different from 1 at the 5% level.
In the example above the case-control study of only 79 subjects produced an odds ratio () that was a very close approximation to the risk ratio () that was obtained from the data in the entire population.
In a matched case-control study, the cell counts represent pairs, not individuals.
In the statistical analysis, only the discordant pairs are important. Cells B and C contribute to the odds ratio in a matched design. Cells A and D do not contribute to to the odds-ratio.
If the risk for disease is increased due to exposure, C will be greater than B. Published formulas for case-control designs provide sample sizes required to determine that a given disease-exposure odds ratio is significantly different from one, adjusting for a potential confounder and possible interaction.
The formulas are extended from one control per case to F controls per case and adjusted for a potential multi-category confounder in unmatched or matched . Feb 01, · The authors propose a method to perform a combined analysis of matched and unmatched case-control studies that is based on an adaptation of logistic regression and can be performed using standard software.
The counts represent PAIRS containing 1 case and 1 control rather than individual cases. Results include Fisher Exact and midP Exact tests and confidence limits for the odds ratio, and the McNemar test and the Pair-Matched Odds Ratio to evaluate whether an association exists between case status and the presence or absence of the risk factor.