Sample size determination is often important step in planning a statistical study.  Studies that are too small sizes of samples may lead to inaccurate results but studies that are too large sizes of samples may waste time and resources. Because the appropriate sample size determination depends on the type of data being collected of the study (categorical or continuous) and the purpose of the study (estimate or test).  Therefore, in this paper we discuss the important factors that affect on calculating the appropriate sample size.  These factors, include: the type of data, level of precision, the confidence level, the degree of variability, effect size, type I error, Type II error, significance level,  power of the test and other factors. Also, we discuss and illustrate the different procedures (formulas) for sample size calculation.