Sample size calculation in physical medicine and rehabilitation: a systematic review of reporting, characteristics, and results in randomized controlled trials

Objective: To assess systematically the reporting of sample size calculation in randomized controlled trials (RCTs) in 5 leading journals in the field of physical medicine and rehabilitation (PM&R).

Data sources: The data source was full reports of RCTs in 5 leading PM&R journals (Journal of Rehabilitation Medicine, Archives of Physical Medicine and Rehabilitation, American Journal of Physical Medicine and Rehabilitation, Clinical Rehabilitation, and Disability and Rehabilitation) between January and December of 1998 and 2008. Articles were identified in Medline.

Study selection: A total of 111 articles met our inclusion criteria, which include RCTs of human studies in the 5 selected journals.

Data extraction: Sample size calculation reporting and trial characteristics were collected for each trial by independent investigators.

Data synthesis: In 2008, 57.3% of articles reported sample size calculation as compared with only 3.4% in 1998. The parameters that were commonly used were a power of 80% and alpha of 5%. Articles often failed to report effect size or effect estimates for sample size calculation. Studies reporting sample size calculation were more likely to describe the main outcome and to have a sample size greater than 50 subjects. The study outcome (positive vs negative) was not associated with the likelihood of sample size reporting. Trial characteristics of the 2 periods (1998 vs 2008) were similar except that in 1998 there were more negative studies compared with 2008.

Conclusions: Although sample size calculation reporting has improved dramatically in 10 years and is comparable with other fields in medicine, it is still not adequate given current publication guidelines.