Approaches to sample size calculation in comparative studies.

The questions 'what should be the minimum study subjects' and 'how these subjects should be enrolled in the study' are two aspects that are considered at the design stage. Regardless of research question and the study design adopted, these two questions are an integral part of any research proposal. No matter how well a study is conducted and statistically analysed using sophisticated statistical methods/softwares, if the study size is inadequate, concluding statements will have inadequate power and such statements may be misleading and harmful to both target population and the scientific community. There is no substitute/shortcut to an adequate study size as no statistical methods are available to adjust for an inadequate study size. Though a larger sample size will result in higher precision in estimates, however, it may not be desirable in terms of cost, time and the efforts. In medical sciences, majority of research questions pertain to comparing outcome measures in two or more than two groups. Focus of this article is on the number of study subjects required in a study and an attempt is made to review the ingredients required to calculate the sample size in situations commonly encountered by researchers in medical sciences. Formulae for the specific situations are provided with worked out examples.