Statistics review 4: sample size calculations

Elise Whitley, Jonathan Ball, Elise Whitley, Jonathan Ball

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.

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.

References

    1. Moher D, Dulberg CS, Wells GA. Statistical power, sample size, and their reporting in randomized controlled trials. JAMA. 1994;272:122–124.
    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.
    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.
    1. Anonymous Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med. 1996;342:1301–1308.
    1. Gattinoni L, Tognoni G, Pesenti A, Taccone P, Mascheroni D, Labarta V, Malacrida R, Di Giulio P, Fumagalli R, Pelosi P, Brazzi L, Latini R, Prone-Supine Study Group Effect of prone positioning on the survival of patients with acute respiratory failure. N Engl J Med. 2001;345:568–573.
    1. Zijlstra JG, Ligtenberg JJ, van der Werf TS. Prone positioning of patients with acute respiratory failure. N Engl J Med. 2002;346:295–297.
    1. Slutsky AS. The acute respiratory distress syndrome, mechanical ventilation, and the prone position. N Engl J Med. 2001;345:610–612.

Source: PubMed

Sponsorzy i współpracownicy

Warunki medyczne

Interwencje lekowe

3
Subskrybuj