Analyzing Industry’s Current Perceptions of Parallel-Group Cluster-Randomized Trials

Image Credit: © Andrii Yalanskyi –

A recent study published by the National Library of Medicine aimed to evaluate researchers’ experiences with the trial design and conduct of parallel-group cluster-randomized trials (CRTs). The authors introduce a potential “open-cohort” design, which may be able to alleviate common challenges seen while utilizing a closed-cohort design.1

“Two accepted designs exist for parallel-group CRTs. Closed-cohort designs follow the same individuals over time with a single recruitment period before randomization, but face challenges in settings with high attrition,” the authors wrote. “(Repeated) cross-sectional designs recruit at one or more timepoints before and/or after randomization, collecting data from different individuals present in the cluster at these timepoints, but are unsuitable for assessment of individual change over time. An ‘open-cohort’ design allows individual follow-up with recruitment before and after cluster-randomization, but little literature exists on acceptability to inform their use in CRTs.”

According to the National Institutes of Health, PubMed abstracts identifying group-randomized trials have increased dramatically over the past decade. Research found that approximately 500 abstracts identified group-randomized trials in 2010, while over 1200 did so in 2020.2

To evaluate the views of researchers’ experiences with CRTs, the study authors utilized qualitative consultation via two expert workshops. Transcripts from the workshops were analyzed using Descriptive Thematic Analysis utilizing inductive and deductive coding.

The predominant challenge identified in current CRT designs was the loss of follow up in certain locations.

“Anticipated attrition may, therefore, force adaptation of the research question when using a [closed cohort] design, narrowing the target population to which inferences can be made,” the study authors wrote. “This is a concern as the research question should drive the trial design rather than vice-versa. Similarly, the [repeated cross-sectional] design, due to its cross-sectional nature, is able to provide cluster-level inference at specific time points. It is generally unsuitable where the research questions involve an assessment of individual change over time.”

The researchers confirmed that current CRT designs are unable to handle the complex research and intervention circumstances found in some trial settings, such as care homes. Participants generally recognized the value of an open-cohort design, citing recruitment post-randomization as a key strength. However, there was uncertainty around when utilizing an open-cohort design would be appropriate.

There were significant differences among the participants’ opinions on statistical and health economic analysis in open-cohort designs. They felt that handling missing data that require further exploration was an area of uncertainty.

According to the authors, this is the first study to explore this topic with trial experts. A limitation of study was that the participants were all United Kingdom-based, so it does not include an international perspective. Most of the roles of the researchers weighed towards statisticians and those working predominantly in care home trials.

“Open-cohort CRT designs hold promise for addressing some of the challenges associated with standard CRT designs. However, there currently remains limited research on such designs to provide clarity around definition and guidance on their application,” the authors concluded.


  1. Surr C, Marsden L, Griffiths A, Cox S, Fossey J, Martin A, Prevost AT, Walshe C, Walwyn R. Researchers’ experiences of the design and conduct challenges associated with parallel-group cluster-randomised trials and views on a novel open-cohort design. PLoS One. 2024 Feb 23;19(2):e0297184. doi: 10.1371/journal.pone.0297184. PMID: 38394190; PMCID: PMC10889884.
  2. PubMed Abstracts Identifying Group-Randomized Trials. National Institutes of Health. Updated December 17, 2021. Accessed March 4, 2024.

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