Step-down multiple tests for comparing treatments with a control in unbalanced one-way layouts.
We show how a well-known multiple step-down significance testing procedure for comparing treatments with a control in balanced one-way layouts can be applied in unbalanced layouts (unequal sample sizes for the treatments). The method we describe has the advantage that it provides p-values, for each treatment versus control comparison, that take account of the multiple step-down testing nature of the procedure. These joint p-values can be used with any value of alpha, the fixed type I family wise error rate bound, that may be specified by the investigator. To determine the p-values, it is necessary to compute a multivariate Student t integral, for which a computer program is available. This procedure is more powerful than the step-down Bonferroni procedure of Holm and the single-step procedure of Dunnett. An example from the pharmaceutical literature is used to illustrate the procedure.
Dunnett
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CW
Department of Clinical Epidemiology & Biostatistics, McMaster University, Hamilton, Ontario, Canada.
Tamhane
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AC
D000818:Animals; D002312:Cardiomyopathy, Hypertrophic / Q000188:drug therapy / Q000503:physiopathology; D005163:Factor Analysis, Statistical; D015999:Multivariate Analysis; D051381:Rats; D012107:Research Design; D013223:Statistics as Topic; D013812:Therapeutics; D013963:Thyroid Hormones / Q000502:physiology
D013963:Thyroid Hormones