Razi Journal of Medical Sciences

One of the questions that most investigators specially physicians encounter when they plan a research project is determining sample sizes. Selecting a sample greater than what is required to obtain the desired results causes wastage of the sources, while selection of very small samples often leads the investigator to the results which have no practical use. Determining number ot the required samples depends on various factors such as type, purpose and domain of action of the study and expected results which all should be considered at the time of planning.

The present article deals with the cases that make it necessary to adopt a great smaple, and the cases that make selection of the samail smples more advisable. Anyway, in the cases that number of the samples is optionally selected, the percentge of problems appeared in research findings will increase and leads to wrong interpretaition or the results.

Meanwhile it should be taken into account that large number of samples cannot be a substitute for incorrect samplins plan.

In recent years, easier methods have been used to determine the required number of samples in addition to the formulas presented in the statistical texts. one of these methods is to prosper the tables and pamphlets that are published by WHO.

The presented tables and examples in the pamphletes are selected in such a way that contain many of the current methods in the health investigations. This not only has the direct practical application for the researchers but also opens a horizon toward the statistical methodology in determining number of the samples.