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How to Define Analytical Populations for a Clinical Study

By John Pezzullo

Analytical populations are precisely defined subsets of the enrolled subjects that are used for different kinds of statistical analysis. Most clinical trials include the following types of analytical populations:

  • The safety population: This group usually consists of all subjects who received at least one dose of any study product (even a placebo) and had at least one subsequent safety-related visit or observation. All safety-related tabulations and analyses are done on the safety population.

  • The intent-to-treat (ITT) population: This population usually consists of all subjects who received any study product. The ITT population is useful for assessing effectiveness — how well the product performs in the real world, where people don’t always take the product as recommended (because of laziness, inconvenience, unpleasant side effects, and so on).

  • The per-protocol (PP) population: This group is usually defined as all subjects who complied with the rules of the study — those people who took the product as prescribed, made all test visits, and didn’t have any serious protocol violations. The PP population is useful for assessing efficacy — how well the product works in an ideal world where everyone takes it as prescribed.

Other special populations may be defined for special kinds of analysis. For example, if the study involves taking a special set of blood samples for pharmacokinetic (PK) calculations, the protocol usually defines a PK population consisting of all subjects who provided suitable PK samples.