A graph of sample sizes for retrospective studies.

Sample sizes for usability studies: additional considerations.

Recently, Virzi (1992) presented data that support three claims regarding sample sizes for usability studies: (1) observing four or five participants will allow a usability practitioner to discover 80% of a product's usability problems, (2) observing additional participants will reveal fewer and fewer new usability problems, and (3) more severe usability problems are easier to detect with the f...

Sample sizes for self-controlled case series studies.

We derive several formulae for the sample size required for a study designed using the self-controlled case series method without age effects. We investigate these formulae by simulation, and identify one based on the signed root likelihood ratio statistic which performs well. We extend this method to allow for age effects, which can have a big impact on the sample size needed. This more genera...

Sample sizes of studies on diagnostic accuracy: literature survey.

OBJECTIVES To determine sample sizes in studies on diagnostic accuracy and the proportion of studies that report calculations of sample size. DESIGN Literature survey. DATA SOURCES All issues of eight leading journals published in 2002. METHODS Sample sizes, number of subgroup analyses, and how often studies reported calculations of sample size were extracted. RESULTS 43 of 8999 article...

Comparing two Formulas of Sample Size Determination for Prevalence Studies

Background and purpose: Sample size and its determination is one of the most important problems in health researches. Calculating sample size for prevalence studies is one of the common questions of sample size topics. Minimum sample size with least complexity is desirable in order to achieve the basic goal of these studies. This study aims to compare two formulas of sample size calculation for...

Sample Sizes for Multilevel Modeling