Document Detail

Properties of permuted-block randomization in clinical trials.
MedLine Citation:
PMID:  3203524     Owner:  NLM     Status:  MEDLINE    
This article describes some of the important statistical properties of the commonly used permuted-block design, also known simply as blocked-randomization. Under a permutation model for statistical tests, proper analyses should employ tests that incorporate the blocking used in the randomization. These include the block-stratified Mantel-Haenszel chi-square test for binary data, the blocked analysis of variance F test, and the blocked nonparametric linear rank test. It is common, however, to ignore the blocking in the analysis. For these tests, it is shown that the size of a test obtained from an analysis incorporating the blocking (say T), versus an analysis ignoring the blocking (say TI), is related to the intrablock correlation coefficient (R) as TI = T(1-R). For blocks of common length 2m, the range of R is from -1/(2m-1) to 1. Thus, if there is a positive intrablock correlation, which is more likely than not for m greater than 1, an analysis ignoring blocking will be unduly conservative. Permutation tests are also presented for the case of stratified analyses within one or more subgroups of patients defined post hoc on the basis of a covariate. This provides a basis for the analysis when responses from some patients are assumed to be missing-at-random. An alternative strategy that requires no assumptions is to perform the analysis using only the subset of complete blocks in which no observations are missing. The Blackwell-Hodges model is used to assess the potential for selection bias induced by investigator attempts to guess which treatment is more likely to be assigned to each incoming patient. In an unmasked trial, the permuted-block design provides substantial potential for selection bias in the comparison of treatments due to the predictability of the assignments that is induced by the requirement of balance within blocks. Further, this bias is not eliminated by the use of random block sizes. We also modify the Blackwell-Hodges model to allow for selection bias only when the investigator is able to discern the next assignment with certainty. This type of bias is reduced by the use of random block sizes and is eliminated only if the possible block sizes are unknown to the investigators. Finally, the Efron model for accidental bias is used to assess the potential for bias in the estimation of treatment effects due to covariate imbalances. For the permuted-block design, the variance of this bias approaches that of complete randomization as the half-block length m----infinity.(ABSTRACT TRUNCATED AT 400 WORDS)
J P Matts; J M Lachin
Related Documents :
9147604 - Comparing two diagnostic tests against the same "gold standard" in the same sample.
17440024 - Blinded trials taken to the test: an analysis of randomized clinical trials that report...
9308134 - Controlling false positive rates in prognostic factor analyses with small samples.
19694294 - The 3d moore-rayleigh test for the quantitative groupwise comparison of mr brain images.
2766814 - Comparison of two-minute incremental threshold loading and maximal loading as measures ...
7784814 - Detecting erythromycin resistance in streptococcus pyogenes: reliability of the disk di...
Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  Controlled clinical trials     Volume:  9     ISSN:  0197-2456     ISO Abbreviation:  Control Clin Trials     Publication Date:  1988 Dec 
Date Detail:
Created Date:  1989-02-03     Completed Date:  1989-02-03     Revised Date:  2007-11-15    
Medline Journal Info:
Nlm Unique ID:  8006242     Medline TA:  Control Clin Trials     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  327-44     Citation Subset:  IM    
Department of Surgery, University of Minnesota, Minneapolis 55414.
Export Citation:
APA/MLA Format     Download EndNote     Download BibTex
MeSH Terms
Analysis of Variance
Clinical Trials as Topic / methods*
Models, Statistical
Random Allocation*
Research Design*
Grant Support
HL-15265/HL/NHLBI NIH HHS; R01-DI-39826//PHS HHS

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

Previous Document:  Properties of simple randomization in clinical trials.
Next Document:  Properties of the urn randomization in clinical trials.