Revista portuguesa de cardiologia : orgão oficial da Sociedade Portuguesa de Cardiologia = Portuguese journal of cardiology : an official journal of the Portuguese Society of Cardiology
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In order to be valid, clinical studies must be methodologically rigorous. The internal validity of a study is of crucial importance: a study is valid if its results are an unbiased estimation of the true result. In this case, the validity is internal because it refers to the group of patients under study and not necessarily different ones (external validity or applicability). ⋯ On the other hand, a study with too large a sample is harder to conduct and more costly. The goal of planning the sample size is to estimate the appropriate number of research subjects for the study. In this paper we will present and discuss the methodological principles underlying calculation of sample size: outcomes, type I and II error, alpha and beta, study power and variability.