This study sought to review the extent to which unequal randomisation has been used in clinical trials of physiotherapy interventions.
The Physiotherapy Evidence Database (PEDro) was searched using these search terms: ratio; unequal* random*; unequal* allocat*, imbalanced random*; imbalanced allocat*; unbalanced random*; unbalanced allocat*, 2:1, 3:1, 4:1, 5:1, 1:2, 1:3, 1:4 and 1:5. Titles and abstracts of the retrieved records were screened to identify trials that used unequal randomisation, and the reasons for using unequal randomisation were extracted in duplicate from those trials.
The searches retrieved 2,903 trials. Fifty-six trials were identified that had used unequal randomisation: median PEDro scale score of 6 (range 5 to 7); median publication year of 2016 (range 1990 to 2024).
These 56 trials represent 0.1% of all physiotherapy trials.
Among these 56 trials, unequal randomisation was used: to increase data on or experience with the experimental intervention (13 trials); to compensate for anticipated imbalanced attrition (9); to facilitate recruitment (4); to enable secondary analyses (4); to minimise ethical issues of allocating participants to a control intervention (3); to reduce costs (3); and for other reasons (6). Six trials reported multiple (2 to 4) reasons, whilst 22 trials did not report a reason for using unequal randomisation. None reported using it to increase data on rare side effects or due to limited availability of the experimental intervention.
Use of unequal randomisation is extremely rare in trials of physiotherapy interventions. In contrast, estimates of the prevalence of unequal randomisation in trials of medical interventions range from 0.8% to 4.7%. Furthermore, it appears that physiotherapists might be able to use unequal randomisation for a wider range of reasons.
Research physiotherapists should consider using unequal randomisation whenever it might improve the cost, speed, recruitment or internal validity of a clinical trial, such as in the situations explained above. When they use it, they should be explicit about their reason(s) for using it. Clinical physiotherapists should be reassured that unequal randomisation does not impair the scientific validity of a randomised trial.
