Using pairwise randomisation to reduce the risk of bias

Abstract

Objective To describe the roles that pairwise randomisation has to prevent selection bias and allow a common outcome measurement point when timing of treatment is uncertain. Study design and setting: A theoretical paper discussing situations where pairwise randomisation might be an appropriate method of allocation. Results When recruitment of participants allows two or more randomisations to occur simultaneously, then pairwise randomisation allows site stratification but prevents site predictability. For evaluations of treatments where timing of treatment is uncertain (e.g. elective surgery), pairwise randomisation allows a common post-treatment evaluation time point. In waiting list designs where length of treatment varies by patient, pairwise randomisation allows differential follow-up across pairs. Conclusion Since pairwise randomisation was described in 2004, to allow unpredictable site stratification, it has been rarely used. However, it can address other methodological issues and should be considered more widely.