Skip to content

sample size

조회 수 25 추천 수 0 2020.10.13 23:13:07





Sample Sizes for Two Independent Samples, Continuous Outcome

In studies where the plan is to perform a test of hypothesis comparing the means of a continuous outcome variable in two independent populations, the hypotheses of interest are:


where μ 1 and μ 2 are the means in the two comparison populations. The formula for determining the sample sizes to ensure that the test has a specified power is:

where ni is the sample size required in each group (i=1,2), α is the selected level of significance and Z 1-α /2 is the value from the standard normal distribution holding 1- α /2 below it, and 1- β is the selected power and Z 1-β is the value from the standard normal distribution holding 1- β below it. ES is the effect size, defined as:

where | μ 1 - μ 2 | is the absolute value of the difference in means between the two groups expected under the alternative hypothesis, H1. σ is the standard deviation of the outcome of interest. Recall from the module on Hypothesis Testing that, when we performed tests of hypothesis comparing the means of two independent groups, we used Sp, the pooled estimate of the common standard deviation, as a measure of variability in the outcome.

Sp is computed as follows:

If data are available on variability of the outcome in each comparison group, then Sp can be computed and used to generate the sample sizes. However, it is more often the case that data on the variability of the outcome are available from only one group, usually the untreated (e.g., placebo control) or unexposed group. When planning a clinical trial to investigate a new drug or procedure, data are often available from other trials that may have involved a placebo or an active control group (i.e., a standard medication or treatment given for the condition under study). The standard deviation of the outcome variable measured in patients assigned to the placebo, control or unexposed group can be used to plan a future trial, as illustrated.  

 Note also that the formula shown above generates sample size estimates for samples of equal size. If a study is planned where different numbers of patients will be assigned or different numbers of patients will comprise the comparison groups, then alternative formulas can be used (see Howell3 for more details).

List of Articles
번호 제목 글쓴이 날짜 조회 수sort
1695 epitope spreading han 2006-05-09 38360
1694 Heinrich Law (1:29:300 Law) WHRIA 2007-08-12 24940
1693 일본 주소 [2] WHRIA 2008-06-28 23940
1692 사주팔자 프로그램 file 한승석 2003-02-16 20266
1691 simple adblock file WHRIA 2012-04-13 18052
1690 도란사민 WHRIA 2011-04-19 18041
1689 탤런트 이윤지씨와 함께 file WHRIA 2010-02-04 16977
1688 penicillin han 2003-12-10 15648
1687 SSH tunnel WHRIA 2007-10-01 15055
1686 geexbox [1] han 2006-12-01 14580
1685 아이피부과 개원 WHRIA 2010-01-18 14383
1684 세무회계 WHRIA 2010-01-31 14364
1683 증명사진 WHRIA 2010-03-14 14289
1682 2만원 WHRIA 2010-02-11 14159
1681 WHRIA WORLD ( 한승석 2002-04-12 14095

Powered by Xpress Engine / Designed by Sketchbook

sketchbook5, 스케치북5

sketchbook5, 스케치북5

나눔글꼴 설치 안내

이 PC에는 나눔글꼴이 설치되어 있지 않습니다.

이 사이트를 나눔글꼴로 보기 위해서는
나눔글꼴을 설치해야 합니다.

설치 취소