Tuyển tập các báo cáo nghiên cứu về y học được đăng trên tạp chí y học General Psychiatry cung cấp cho các bạn kiến thức về ngành y đề tài: Statistics review 4: Sample size calculations. | Available online http content 6 4 335 Review Statistics review 4 Sample size calculations Elise Whitley1 and Jonathan Ball2 Lecturer in Medical Statistics University of Bristol Bristol UK 2Lecturer in Intensive Care Medicine St George s Hospital Medical School London UK Correspondence Editorial Office Critical Care editorial@ Published online 10 May 2002 Critical Care 2002 6 335-341 This article is online at http content 6 4 335 2002 BioMed Central Ltd Print ISSN 1364-8535 Online ISSN 1466-609X 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. Keywords statistical power sample size Previous reviews in this series introduced confidence intervals and P values. Both of these have been shown to depend strongly on the size of the study sample in question with larger samples generally resulting in narrower confidence intervals and smaller P values. The question of how large a study should ideally be is therefore an important one but it is all too often neglected in practice. The present review provides some simple guidelines on how best to choose an appropriate sample size. Research studies are conducted with many different aims in mind. A study may be conducted to establish the difference or conversely the similarity between two groups defined in terms of a particular risk factor or treatment regimen. Alternatively it may be conducted to estimate some quantity for example the prevalence of disease in a specified population with a given degree of precision. Regardless of the motivation for the study it is essential that it be of an appropriate size to achieve its aims. The most common aim is probably that of determining .
