Case Control Study

Lead Author(s): Jeff Martin, MD

Case Control Design

The main advantage of case-control designs is that it allows you to sample the experience of the study base most efficiently. The reason to do this is to conserve resources, something that is becoming more and more important these days as funding is drying up. A typical example is when expensive testing on stored biological samples are required for an analysis. It is often prohibitively costly to test everyone in the cohort.

Sampling Cases Within a Cohort

Given that all the cases are diagnosed, how would you sample controls from this cohort for a case-control study?

Think of the selection of cases and controls as occurring from a cohort as in the diagram below.

Three Ways to Sample Controls Within a Cohort

Summary of Case-Control Sampling

Design Sampling Measure of Association
Case-cohort Entire cohort at baseline risk ratio
Incidence-density Non-cases at time of diagnosis rate ratio
Prevalent Case Control Non-cases at single point in time odds ratio

Case-control design obtains a sample of the denominator rather than entire denominator

Important Features of a Case Control Study

(1) Primary versus Secondary Study Base: (2) Incident versus Prevalent Sampling:

Prevalent Sampling in a Case-Control Study

(1) UCSF Study of glioma patients is an example of prevalent sampling.

The patients are under treatment at UCSF during study period (2) Kaiser Permanente Study of Sigmoidoscopy is another example of prevalent sampling.

Critical Features of Good Case-Control Design

Clearly identifiable study base (preferably a primary study base)

Cases: all, or random sample, of incident diagnoses in the study base

Controls: an unbiased sample of study base to estimate exposure prevalence in non-cases

Measurements preferably based on records or stored biological samples rather than recall

Case-control studies with all of these design features are a strong and valid study design that can produce results as convincing as any other type of observational study.

Measures of Association in a Case Control Study

The odds ratio is the only measure of association available in case-control design.

The odds ratio of disease in the exposed and unexposed The odds ratio of exposure in the diseased and not diseased that is actually measured in a case-control study, Fortunately, the odds ratio approximates the risk ratio and approximates the rate ratio in a case control study.

Common Misunderstandings about Case Control Studies

Reasons for Choosing Case Control Study Design

Case Control Study Design Recommendations

Historically the two chief weaknesses of case-control studies have been inappropriate selection of a control group and poor measurement of exposure.

Poor measurement often occurs from using questionnaire recall of exposures.

If those weaknesses can be avoided, case-control can be a solid valid study design.