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Review
. 2002 Aug;6(4):335-41.
doi: 10.1186/cc1521. Epub 2002 May 10.

Statistics review 4: sample size calculations

Elise Whitley  1 , Jonathan Ball
Affiliations
Review

Statistics review 4: sample size calculations

Elise Whitley et al. Crit Care. 2002 Aug.

Abstract

The present review introduces the notion of statistical power and the hazard of under-powered studies. The problem of how to calculate an ideal sample size is also discussed within the context of factors that affect power, and specific methods for the calculation of sample size are presented for two common scenarios, along with extensions to the simplest case.

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Figures

Figure 1
Figure 1
Nomogram for calculating sample size or power. Reproduced from Altman [5], with permission.
Figure 2
Figure 2
Nomogram showing sample size calculation for a standardized difference of 0.78 and 80% power.
Figure 3
Figure 3
Nomogram showing sample size calculation for a standardized difference of 0.78 and 95% power.
Figure 4
Figure 4
Nomogram showing sample size calculation for standardized difference of 0.188 and 90% power.
Figure 5
Figure 5
Nomogram showing the statistical power for a standardized difference of 0.188 and a total sample size of 861.
Figure 6
Figure 6
Nomogram showing the statistical power for a standardized difference of 0.095 and a total sample size of 304.
See this image and copyright information in PMC

References

    1. Moher D, Dulberg CS, Wells GA. Statistical power, sample size, and their reporting in randomized controlled trials. JAMA. 1994;272:122–124. - PubMed
    1. Machin D, Campbell MJ, Fayers P, Pinol A. Sample Size Tables for Clinical Studies Oxford, UK: Blackwell Science Ltd; 1987.
    1. Rivers E, Nguyen B, Havstad S, Ressler J, Muzzin A, Knoblich B, Peterson E, Tomlanovich M. Early goal-directed therapy in the treatment of severe sepsis and septic shock. N Engl J Med. 2001;345:1368–1377. - PubMed
    1. Altman DG. Practical Statistics for Medical Research London, UK; Chapman & Hall; 1991.
    1. Altman DG. How large a sample? In: Gore SM, Altman DG, editor. Statistics in Practice. London, UK: British Medical Association; 1982.