Comparing the Means of Several Groups

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Abstract
To explain statistical methods for comparing the means of several groups, this chapter focuses on examples from 50 Original Articles published in the New England Journal of Medicine in 1978 and 1979 and includes a follow-up study of Volume 321 (July through December 1989). Although medical authors often present comparisons of the means of several groups, the most common method of analysis, multiple t-tests, is usually a poor choice because it does not take into account multiplicity. Which method of analysis is appropriate depends on what questions the investigators wish to ask. If the investigators want to identify groups under study that differ from the rest, they need a different method from the one required if they wish simply to decide whether the groups share a common mean. More complicated questions about the group means call for more sophisticated techniques. Of the 50 Journal articles examined, 27 (54 percent) could have used more appropriate statistical methods to analyze the differences between group means. Analysis of variance and multiple comparison methods including the Bonferroni, Duncan, Newman-Keuls, Scheffe, and Tukey methods offer useful methods for comparing means.

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