Case-Control Design - Sampling at Baseline

Special type of case-control study: Case cohort design

Lead Editor(s): Jeff Martin, MD

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

Definition of Sampling for Case Cohort Design

One way to sample controls within a cohort is to take a random sample of a cohort at baseline. It is relatively new and has not been frequently used. It was first described by the statistician Ross Prentice in 1986.

Diagram of Random Sampling at Baseline

As you can see in the diagram below, controls are chosen at baseline from the hypothetical cohort.

Case-cohort: sample baseline of cohort

0527_1_b_N.JPG is sampled randomly from baseline.


The case-cohort design is distinguished by taking a random sample of the study cohort at baseline. All of the cases are included in the study (although also could be a random sample),

Sampling Controls at Baseline

Most people think that the best way to sample controls is to wait until the end of follow-up so that the investigator can be sure they will not be cases.

Becoming a case is an artifact of the follow-up period of the cohort. When we are looking for (i.e., sampling) controls, we do not necessarily have to guarantee that these are subjects will never become cases. All that is needed is to be sure that they are not cases at the time of control sampling. CONTROL GROUP is a random sample of the cohort at baseline.

Case-Based Estimate of Exposed and Unexposed

The case-cohort design begins with the cases, so like other types of case-control sampling it is case-based.

This random sample of the baseline population will estimate the proportion exposed and unexposed in the entire study base.

Advantages of Case-Cohort Design

The case-control design becomes a good alternative to the other types of case-control sampling if:

How OR = Risk Ratio in a Case-Cohort Design

The graphic below shows the notational framework of the 2x2 table to illustrate the properties of the odds ratio.

There is a:
  1. A cohort of exposed and unexposed cases
  2. Estimate of the proportion of exposed and unexposed in the whole cohort (controls)

The critical point is that the controls in the case cohort design estimate the proportions exposed and unexposed in the whole cohort, When the odds ratio of exposure in the cases and controls is formed and the notation from a cohort is substituted for the cells in the 2x2 table, the resulting ratio is the same as the risk ratio.

The odds of exposure among those with disease is easily formed simply by taking: