Sample size in clinical research, the number we need -

In the era of evidence based medicine where we should base our clinical practice on the available evidence of growing medical literatures, large number of available published literatures do not have enough sample size to detect their desirable primary or secondary outcome. It is surprised that even in journal with high impact factors the sample size calculation is still inadequate, often erroneous and based on assumption which are inaccurate. Sample size calculation is always a difficult issue, which should be address diligently before commencement of the study. The objective of the current narrative review is to illustrate the methodology of sample size calculation for comparative and descriptive research, which will be most useful for the beginners.