Sampling - ANCHORS

Lead Author(s): Jeff Martin, MD

Definition of Sampling

Sampling is obvious in some study designs but less so in others, such as case-control designs, but it is the key to understanding a properly designed case-control study.

Random Sampling

Incident versus Prevalent Sampling

Incident versus Prevalent sampling refers to how the cases and controls are sampled (both types of sampling can be done either in a primary or a secondary study base).

Incidence Sampling of Case and Controls Is Preferable

One of the major criticisms of the case-control design is that it is retrospective. This is correct in that the study is carried out after the disease experience of the study base has already occurred.

In terms of study validity though, the key question is not when is the study being carried out, but when were the measurements made and how good are they?

The weakness in measuring predictors in a case-control study comes when subject recall is relied on. The strongest case-control studies look for measurements that were made in the past before the disease diagnosis; for example, measurements captured in medical records in an HMO.