Risk of selection bias in randomised trials

Background: Selection bias occurs when recruiters selectively enrol patients into the trial based on what the next treatment allocation is likely to be. This can occur even if appropriate allocation concealment is used if recruiters can guess the next treatment assignment with some degree of accuracy. This typically occurs in unblinded trials when restricted randomisation is implemented to force the number of patients in each arm or within each centre to be the same. Several methods to reduce the risk of selection bias have been suggested; however, it is unclear how often these techniques are used in practice. Methods: We performed a review of published trials which were not blinded to assess whether they utilised methods for reducing the risk of selection bias. We assessed the following techniques: (a) blinding of recruiters; (b) use of simple randomisation; (c) avoidance of stratification by site when restricted randomisation is used; (d) avoidance of permuted blocks if stratification by site is used; and (e) incorporation of prognostic covariates into the randomisation procedure when restricted randomisation is used. We included parallel group, individually randomised phase III trials published in four general medical journals (BMJ, Journal of the American Medical Association, The Lancet, and New England Journal of Medicine) in 2010. Results: We identified 152 eligible trials. Most trials (98%) provided no information on whether recruiters were blind to previous treatment allocations. Only 3% of trials used simple randomisation; 63% used some form of restricted randomisation, and 35% did not state the method of randomisation. Overall, 44% of trials were stratified by site of recruitment; 27% were not, and 29% did not report this information. Most trials that did stratify by site of recruitment used permuted blocks (58%), and only 15% reported using random block sizes. Many trials that used restricted randomisation also included prognostic covariates in the randomisation procedure (56%). Conclusions: The risk of selection bias could not be ascertained for most trials due to poor reporting. Many trials which did provide details on the randomisation procedure were at risk of selection bias due to a poorly chosen randomisation methods. Techniques to reduce the risk of selection bias should be more widely implemented

Retrospective analyses versus RCTs: comparing like with like?

Ralf Baron,1 Lieven Nils Kennes,2 Christian Elling31Division of Neurological Pain Research and Therapy, Department of Neurology, University Hospital of Schleswig-Holstein, Kiel Campus, Kiel, 2Department of Economics and Business Administration, University of Applied Sciences Stralsund, Stralsund, 3Grünenthal GmbH, Medical Affairs Europe and North America, Aachen, GermanyIn their recent retrospective analysis assessing oxycodone/naloxone (OXN) vs. tapentadol (TAP) treatment for chronic low-back pain with a neuropathic component, Ueberall and Mueller-Schwefe1 compare their results to the findings of an earlier phase 3b/4 study.2 In our opinion, a proper comparison to the prospective, randomized, controlled, open-label study by Baron and colleagues is scientifically not appropriate. Although Ueberall and Mueller-Schwefe use the terms “prospective,” “randomly,” and “blinded” and refer to the PROBE design (prospective, randomized, open-label, blinded endpoint),3 their database study is retrospective, nonrandomized, and nonblinded with the treatment choice left to the discretion of the physicians. In this context, the use of the term “intention-to-treat (ITT) population” is inappropriate because ITT is unambiguously defined as including all randomized subjects and thus inseparable from true randomization (ICH E9).4View original paper by Ueberall and Mueller-Schwefe