Minimizing predictability while retaining balance through the use of less restrictive randomization procedures

Vance W. Berger, Anastasia Ivanova, Maria Deloria Knoll

Research output: Contribution to journalArticlepeer-review

136 Scopus citations


The interpretation of between-group comparisons is facilitated by the creation of treatment groups that are similar to each other in baseline composition. To prevent treatment effects from being confounded with time effects, most trials use restricted randomization to force balance. An unintended consequence of these restrictions is that they create patterns that allow for the prediction of future treatment allocations, and hence selection bias, especially in unmasked trials. In fact, the more restrictive the allocation procedure, the greater the potential for selection bias. It was decided, in the context of a recent clinical trial comparing two dosing schedules of paclitaxel and carboplatin for advanced stage IIIB/IV non-small-cell lung cancer, that the randomized block procedure could not simultaneously protect sufficiently against both selection and chronological bias. In this paper we detail our development of the maximal procedure. The maximal procedure takes as input the extent of chronological bias allowed by the randomized block procedure, then matches it, but does so with fewer restrictions. This feature makes the maximal procedure more resistant to selection bias than the randomized block procedure is. Published in 2003 by John Wiley & Sons, Ltd.

Original languageEnglish (US)
Pages (from-to)3017-3028
Number of pages12
JournalStatistics in Medicine
Issue number19
StatePublished - Oct 15 2003
Externally publishedYes


  • Allocation concealment
  • Baseline imbalance
  • Chronological bias
  • Clinical trials
  • Confounding
  • Randomized block procedure
  • Restricted randomization
  • Selection bias

ASJC Scopus subject areas

  • Epidemiology
  • Statistics and Probability


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