The Role of Random Allocation in Randomized Controlled Trials: Distinguishing Selection Bias from Baseline Imbalance

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Allyn Fives


Background: This paper addresses one threat to the internal validity of a randomized controlled trial (RCT), selection bias. Many authors argue that random allocation is used to ensure baseline equality between study conditions in a given study and that statistically significant differences at pretest mean that randomisation has failed.


Purpose: The purpose of this study was to clarify the role of random allocation in an RCT study. Is the role of random allocation to protect against selection bias? And does it have a further role, namely to ensure baseline equality and the absence of statistically significant differences between study conditions at pretest?


Setting: The participants for this study were 229 children in 1st and 2nd grade and data were collected as part of an RCT evaluation of a volunteer reading programme piloted in Ireland, Wizards of Words (WoW).


Intervention: Not applicable.


Research design: The allocation procedure adopted in this study was stratified and blocked random allocation.


Data collection and analysis: Data were collected using standardised and criterion-referenced tests of reading ability. Data were collected by qualified Speech and Language Therapists. Independent-samples t-tests were used to analyse pretest data.


Findings: The role of random allocation is to protect against selection bias, and statistically significant baseline differences can result even when random allocation has been successful. Whether or not random allocation has been successful is determined by the generation of the random allocation sequence and the steps taken to ensure its concealment. The size of differences between study conditions at pretest can be important for the analysis of posttest data but does not by itself determine whether random allocation was successful. In addition, there are serious concerns about the appropriateness of tests of significance when comparing two study conditions at pretest.

Keywords: baseline equality; baseline imbalance; random allocation; selection bias; test of statistical significance


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Fives, A. (2013). The Role of Random Allocation in Randomized Controlled Trials: Distinguishing Selection Bias from Baseline Imbalance. Journal of MultiDisciplinary Evaluation, 9(20), 33–42. Retrieved from