Sample sizes for prevention trials have been too small

Fred Ederer, Timothy R. Church, Jack S. Mandel

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37 Scopus citations


Planners of several large prevention trials have overestimated the expected incidence of events in the control group, largely because they failed either to recognize or to adequately correct for various effects of population selection. Consequently, the studies have been too small in size or too short in duration to achieve their stated objectives. The selection effects include those engendered by the choice of the target population, the self-selection of volunteers, and protocol exclusions. This paper presents a taxonomy of these effects and the likely direction of their influence on the incidence of events and on mortality rates from other causes. Little information is available to help samplesize planners in adjusting for these effects. A few studies have provided information on the extent to which control group incidence rates have fallen short of expectations. In particular, researchers from the University of Minnesota's Colon Cancer Control Study have provided a detailed comparison of event incidence and all-cause mortality rates with general population rates (Am J Epidemiol 1993; 137: 797-810). Other studies should publish similarly detailed information to assist sample-size planners of prevention trials. Until more information is published, this paper provides preliminary guidelines for prevention trial sample-size planning.

Original languageEnglish (US)
Pages (from-to)787-796
Number of pages10
JournalAmerican journal of epidemiology
Issue number7
StatePublished - Apr 1 1993

Bibliographical note

Funding Information:
Received for publication August 12, 1991, and in final form October 13, 1992. Abbreviations: MRRT, Multiple Risk Factor Intervention Trial; SMR, standardized mortality ratio. ' School of Public Health, University of Minnesota, Minneapolis, MN. 2 The EMMES Corporation, Potomac, MD. Repnnt requests to Fred Ederer, The EMMES Corporation, 11325 Seven Locks Road, Suite 214, Potomac, MD 20854. This research was supported by National Cancer Institute contract NO1-CB-610O5. The authors benefited from criticisms of an earlier draft of the manuscript made by Drs. James D Neaton and Phdip C. Prorok.


  • Epidemiologic methods
  • Mortality
  • Randomized controlled trials
  • Selection bias
  • Study design


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