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Pharm Stat. 2013 Sep-Oct;12(5):268-74. doi: 10.1002/pst.1580. Epub 2013 Jun 25.

Subject-level matching for imbalance in cluster randomized trials with a small number of clusters.

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Department of Psychiatry, Weill Cornell Medical College, New York, NY, USA.


In a cluster randomized controlled trial (RCT), the number of randomized units is typically considerably smaller than in trials where the unit of randomization is the patient. If the number of randomized clusters is small, there is a reasonable chance of baseline imbalance between the experimental and control groups. This imbalance threatens the validity of inferences regarding post-treatment intervention effects unless an appropriate statistical adjustment is used. Here, we consider application of the propensity score adjustment for cluster RCTs. For the purpose of illustration, we apply the propensity adjustment to a cluster RCT that evaluated an intervention to reduce suicidal ideation and depression. This approach to adjusting imbalance had considerable bearing on the interpretation of results. A simulation study demonstrates that the propensity adjustment reduced well over 90% of the bias seen in unadjusted models for the specifications examined.


bias reduction; cluster randomized controlled trials; propensity adjustment

[Indexed for MEDLINE]

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