Results Summary

What was the project about?

A stepped-wedge study is a type of clinical trial that can help researchers learn which treatments are effective. Researchers compare groups of patients, such as patients at different clinics, before and after they receive a new treatment. All groups start with usual care. Then, researchers assign groups by chance to the new treatment at different times during the study. This process continues until all groups receive the new treatment.

Because a stepped-wedge study can take a long time, patients may leave before the study ends, resulting in missing data. Missing data make it hard to know if the new treatment works. To address this problem, researchers need methods to design such studies and know how many patients to enroll.

In this project, the research team developed a new method to find out how many patients should be in stepped-wedge studies to get accurate results.

What did the research team do?

To create the new method, the research team used a statistical approach called generalized estimating equation. This approach uses data from different groups over time to estimate how the treatment will work.

Then the research team used data from real stepped-wedge studies to create different scenarios. For example, scenarios had different health outcomes and varying patterns of missing data. The team tested how the new method worked in each scenario.

Patients, clinical researchers, and people working in health systems helped design the method.

What were the results?

The new method helped the research team find out the number of patients to include in different stepped-wedge design scenarios. For instance, the method helped them figure out how many patients should be included when the group sizes changed after some patients left the study.

Compared with current methods, the new method worked well even when data didn’t meet all the requirements for the statistical approach.

The research team created a computer program to use the method.

What were the limits of the project?

The new method assumes that data are missing by chance and patients who leave a study aren’t different from patients who stay. It may not work if data aren’t missing by chance, such as if patients with a certain health problem drop out of a study. The study developed the new method only for stepped-wedge trials with a single treatment.

Future studies can develop methods to help design stepped-wedge studies with more treatments.

How can people use the results?

Researchers can use the new method to find out how many patients to include when designing stepped-wedge studies to compare treatments.

Final Research Report

This project's final research report is expected to be available by December 2022.

Peer-Review Summary

Peer review of PCORI-funded research helps make sure the report presents complete, balanced, and useful information about the research. It also assesses how the project addressed PCORI’s Methodology Standards. During peer review, experts read a draft report of the research and provide comments about the report. These experts may include a scientist focused on the research topic, a specialist in research methods, a patient or caregiver, and a healthcare professional. These reviewers cannot have conflicts of interest with the study.

The peer reviewers point out where the draft report may need revision. For example, they may suggest ways to improve descriptions of the conduct of the study or to clarify the connection between results and conclusions. Sometimes, awardees revise their draft reports twice or more to address all of the reviewers’ comments. 

Peer reviewers commented and the researchers made changes or provided responses. Those comments and responses included the following:

  • The reviewers noted that not all of the cross-sectional designs discussed in the report would result in all participants receiving the intervention, despite the researchers’ claims that this is a particular advantage of stepped-wedge research designs like those discussed in the report. The researchers revised the report to specify that all participants would receive the intervention only in closed-cohort, stepped-wedge trials where additional participants are not added to the cohort over time.
  • The reviewers asked the researchers to describe further how suggestions from their stakeholder advisory panel tied into the sample size and analytic design research in this study. The researchers explained that stakeholder input was not necessarily directly related to sample size calculation or stepped-wedge methods, but to how this research could affect the timeliness or usefulness of study results.
  • The reviewers questioned if the methods developed were based on independence estimating equations and the researchers were inconsistent in their acknowledgments of the potential limitations using this approach. The researchers clarified the report by referring to generalized estimating equations instead of independence estimating equations throughout. They also provided additional simulation results comparing their method using generalized estimating equations to other analytic methods and demonstrating that the researchers’ methods are robust, even in cases where the distribution of scores does not follow a normal distribution. 

Conflict of Interest Disclosures

Project Information

Song Zhang, PhD
Chul Ahn, PhD
University of Texas Southwestern Medical Center at Dallas
Develop Novel Design Methods for Pragmatic Stepped-wedge Cluster Trials with Patient-Centered Outcomes

Key Dates

August 2017
December 2021

Study Registration Information


Has Results
Award Type
Health Conditions Health Conditions These are the broad terms we use to categorize our funded research studies; specific diseases or conditions are included within the appropriate larger category. Note: not all of our funded projects focus on a single disease or condition; some touch on multiple diseases or conditions, research methods, or broader health system interventions. Such projects won’t be listed by a primary disease/condition and so won’t appear if you use this filter tool to find them. View Glossary
Intervention Strategy Intervention Strategies PCORI funds comparative clinical effectiveness research (CER) studies that compare two or more options or approaches to health care, or that compare different ways of delivering or receiving care. View Glossary
State State The state where the project originates, or where the primary institution or organization is located. View Glossary
Last updated: October 20, 2022