Although PCTs do not necessarily require a specific statistical design approach, both the kinds of questions PCTs are designed to answer and the settings in which they take place may tend to favor certain approaches, such as cluster randomization. The nature of the interventions they seek to test, which may involve healthcare delivery changes, might be better implemented through randomization at the practice, clinic, or even hospital level.

In the following sections, we examine key considerations in statistical study design and analysis for PCTs.

Watch the video module: Choosing the Right Study Design

Three Kinds of Randomized Trials

A key design factor for PCTs is the choice of unit of randomization. There are 3 kinds of randomized trials:

In traditional RCTs, the unit of randomization is generally the individual trial participant, and each individual is randomly assigned to receive an experimental intervention, a comparator therapy, or a placebo. There is no interaction among trial participants after randomization. Most drug trials are traditional RCTs.

In individually randomized group treatment trials (IRGTs), like traditional RCTs, the unit of randomization is the individual trial participant, and each individual is randomly assigned to a study condition. However, there is interaction among trial participants after randomization in 1 or more of the study conditions. Many surgical trials and behavioral trials are IRGTs. These trials are sometimes called "partially nested" or "partially clustered" designs.

In cluster randomized trials (CRTs), randomization takes place at the level of the physician, practice, hospital, health system, city block, or other unit that comprises multiple patients or other participants. The groups are randomly assigned to study conditions, and there is interaction among members of the same group before and after randomization. Many trials conducted in communities, worksites, schools, and other settings are CRTs. CRTs are sometimes called "group randomized trials" or "community trials." There are 2 kinds of CRTs:

• Parallel CRTs: In a parallel CRT, there are parallel intervention and control conditions throughout the trial with no crossover.
• Stepped-wedge CRTs: In a stepped-wedge CRT, all groups start the trial in the control condition. The groups cross over to the intervention condition in random order and on a staggered schedule. All groups receive the intervention before the end of the trial.