title: Sample size calculations.
abstract: The sample size is the number of patients or other experimental units that need to be included in a study to answer the research question. Pre-study calculation of the sample size is important; if a sample size is too small, one will not be able to detect an effect, while a sample that is too large may be a waste of time and money. Methods to calculate the sample size are explained in statistical textbooks, but because there are many different formulas available, it can be difficult for investigators to decide which method to use. Moreover, these calculations are prone to errors, because small changes in the selected parameters can lead to large differences in the sample size. This paper explains the basic principles of sample size calculations and demonstrates how to perform such a calculation for a simple study design.
authors.affiliation: ERA-EDTA Registry, Department of Medical Informatics, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands. m.noordzij@amc.uva.nl
authors.forename: Marlies
authors.initials: M
authors.lastname: Noordzij
authors.forename: Friedo W
authors.initials: FW
authors.lastname: Dekker
authors.forename: Carmine
authors.initials: C
authors.lastname: Zoccali
authors.forename: Kitty J
authors.initials: KJ
authors.lastname: Jager
mesh_terms: D002986:Clinical Trials as Topic / Q000379:methods*; D006801:Humans; D055641:Mathematical Concepts; D012107:Research Design; D018401:Sample Size