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Psychol Methods. 2003 Dec;8(4):524-34.

Should providers of treatment be regarded as a random factor? If it ain't broke, don't "fix" it: a comment on Siemer and Joormann (2003).

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Department of Educational Psychology, University of Wisconsin-Madison, Madison, WI 53706-1796, USA.


In their criticism of B. E. Wampold and R. C. Serlin's analysis of treatment effects in nested designs, M. Siemer and J. Joormann argued that providers of services should be considered a fixed factor because typically providers are neither randomly selected from a population of providers nor randomly assigned to treatments, and statistical power to detect treatment effects is greater in the fixed than in the mixed model. The authors of the present article argue that if providers are considered fixed, conclusions about the treatment must be conditioned on the specific providers in the study, and they show that in this case generalizing beyond these providers incurs inflated Type I error rates.

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