Journal
STATISTICAL METHODS IN MEDICAL RESEARCH
Volume 26, Issue 5, Pages 2376-2388Publisher
SAGE PUBLICATIONS LTD
DOI: 10.1177/0962280215597716
Keywords
Clinical trial; adaptive randomization; bias adjusted estimation; Horvitz-Thompson estimator; inverse probability weighting; Rao-Blackwellization
Categories
Funding
- MRC Methodology research fellowship [MR/L012286/1]
- Claudia Adams Barr Program in Innovative Cancer Research
- MRC [MC_UU_00002/3, MC_UU_12013/9, MC_UU_12013/1, MC_UP_1302/2] Funding Source: UKRI
- Medical Research Council [MC_UU_12013/9, MC_UP_1302/2, MC_UU_00002/3, MC_UU_12013/1, MR/N501906/1] Funding Source: researchfish
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Bayesian adaptive trials have the defining feature that the probability of randomization to a particular treatment arm can change as information becomes available as to its true worth. However, there is still a general reluctance to implement such designs in many clinical settings. One area of concern is that their frequentist operating characteristics are poor or, at least, poorly understood. We investigate the bias induced in the maximum likelihood estimate of a response probability parameter, p, for binary outcome by the process of adaptive randomization. We discover that it is small in magnitude and, under mild assumptions, can only be negative - causing one's estimate to be closer to zero on average than the truth. A simple unbiased estimator for p is obtained, but it is shown to have a large mean squared error. Two approaches are therefore explored to improve its precision based on inverse probability weighting and Rao-Blackwellization. We illustrate these estimation strategies using two well-known designs from the literature.
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