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Summary Sample-size determination is often an important step in planning an epidemiological study. There are several approaches to determining sample size. It depends on the type of the study. Descriptive, observational and randomized controlled studies have different formulas to calculate sample size. In this article, we discuss the formulas that can help to estimate sample size in an epidemiological trial. We present a few examples from clinical practice, which may contribute to the understanding of this problem.Keywords: sample size determination Determining an appropriate sample size for a clinical trial is an essential step in the statistical design of the pro-ject. An adequate sample size helps ensure that the stu-dy will yield reliable information, regardless of whether the ultimate data suggest a clinically important difference between the treatments being studied, or the study is in-tended to measure the accuracy of a diagnostic test or the incidence of a disease. Unfortunately, many studies pub-lished in medical literature are conducted with inadequate sample sizes, making the interpretation of negative results difficult.Conductingastudywithaninadequatesamplesize is not only futile, it is also unethical. Exposing pa-tients to the risks inherent in a research is justifiableon-ly if there is a realistic possibility that the results will be-nefitthosesubjects,futuresubjects,orleadtosubstantialscientificprogress.How many individuals will I need to study? This ques-tion iscommonly asked by a clinical investigator and ex-poses oneof many issues that are best to be settled before actually carryingout a study. Consultation with a statisti-cian is worthwhilein addressing many issues of study de-sign, but a statisticianis not always readily available.Sample Size (n) is the number of individuals in a group under study. The larger the sample size, the grea-ter the precision and, thus, power for a given study de-sign to detect an effect of a given size. For statisticians, an n > 30 is usually sufficientfortheCentralLimitTheo-rem to hold so that normal theory approximations can be used for measures such as the standard error of the mean. However, this sample size (n = 30) is unrelated to the cli-nicians’ objective of detecting biologically significantef-fects, which determines the specificsamplesizeneededfor a specificstudy[1].