A new case study from the NIH Pragmatic Trials Collaboratory highlights an interim reassessment of sample size during an ongoing cluster randomized trial. The case study was published this week in the Living Textbook of Pragmatic Clinical Trials.

Researchers in cluster randomized trials must account for potential correlation between clusters in the design and analysis of their trial by estimating the intraclass correlation when calculating the target sample size. Often they use preliminary data from the planned enrollment sites to estimate the correlation. However, when preliminary data are unavailable at the time of study design, they may use interim data collected during the trial itself to reassess the trial’s sample size.

The contributors of the case study focus on FM-TIPS, an NIH Collaboratory Trial, to describe an approach to conducting an interim reassessment of sample size in an ongoing trial. Read the full case study.

FM-TIPS is examining whether the addition of transcutaneous electrical nerve stimulation to routine physical therapy improves movement-evoked pain compared with physical therapy alone among patients with fibromyalgia. The trial is supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases through the NIH HEAL Initiative. Learn more about FM-TIPS.

The contributors of the case study include members of the FM-TIPS study team and leaders of the NIH Collaboratory’s Biostatistics and Study Design Core. David-Erick Lafontant is a statistician, Bridget Zimmerman is a clinical professor of biostatistics, and Emine Bayman is an associate professor of biostatistics—all at the University of Iowa. Megan McCabe is an assistant professor of biostatistics at the University of Alabama at Birmingham. Patrick Heagerty is a professor of biostatistics at the University of Washington. Liz Turner is an associate professor of biostatistics and bioinformatics at Duke University.