title: [How many subjects do I need to power my study?].
abstract: The article presents a tool that helps answer the question How many subjects do I need to power my study? We show how to determine sample size in observational epidemiological studies and provide examples of application using the statistical package Epidat, which is a shareware program developed under the auspices of the Pan American Health Organization, the Galician Board of Health and the University CES of Colombia. Examples of calculation of sample size for prevalence studies (cross-sectional), case-control studies and cohort studies are given.
authors.affiliation: Departamento de Salud Pública  CIGES, Facultad de Medicina, Universidad de la Frontera, Chile. Address: Montt #112, Temuco, Chile. Email: sergio.munoz.n@ufrontera.cl.
authors.forename: Sergio R
authors.initials: SR
authors.lastname: Muñoz Navarro
mesh_terms: D003627:Data Interpretation, Statistical; D006801:Humans; D012107:Research Design; D018401:Sample Size