Sample size determination

Sample size determination is an important step in conducting research since it should provide the necessary information about the population of the study. In case the sample size is smaller than expected, the obtained results of the study cannot be generalized to the target population. On the other hand, the larger the sample size the more precise the findings. However, it is not usually possible to have a large sample size due to research limitations, such as cost, time, or the convenience of collecting the data. There are different approaches to calculate the sample size:

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  1. Performing a pilot study: Pilot studies are conducted similar to the main study with a smaller sample size. In this regard, the sample size of a pilot trial is estimated at 3-5% of the samples included in the main trial. In fact, pilot studies aim to determine the probable problems a study might have and help the calculation of sample size in the main trial.
  2. Using the literature: Researchers can use the estimated sample size of previous studies to determine the sample size of their own research. However, the researchers cannot use the exact amount of the sample size of studies in the literature. They can use standard deviation and mean to estimate their sample size based on those available in the literature.
  3. Using experts’ opinions: This method is applicable when there is no previous study in the field and also there is no possibility of conducting a pilot study. In this regard, the researchers require the experts’ opinions to estimate the sample size. However, this method may provide the imprecise estimation of the sample size.

Regardless of the employed method of sample size determination, it is worth considering the following points:

  1. Sample size determination depends on the type of the main variables of the study whether qualitative or quantitative. In fact, the smaller size of the sample is required when the variables are qualitative.
  2. Smaller sample size is required in interventional studies when the intervention is strong enough.
  3. Homogeneity of the samples is another factor affecting the sample size. The more homogenous the samples, the larger the sample size.

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