[Types of studies, power of study and choice of test]

In scientific research different study designs are used to provide answers to different questions (e.g., What is the cause of a disease? Is medication A better than medication B? What is the prevalence of a disease?). Cross-sectional study is appropriate when we are interested in the prevalence of a disease or certain feature, cohort study provides answer to the incidence or relative risk, whereas case-control study is conducted to obtain odds ratio. While these studies are observational, randomized control trial is the only experimental study design which can give answers to questions of causative relationships between variables as where researcher himself or herself assigns subjects to different groups (experimental or control). Power of a study represents the probability of finding a difference that exists in a population. It depends on the chosen level of significance, difference that we look for (effect size), variability of the measured variables, and sample size. Most often sample size is the only element under direct researcher's control. Therefore, methods have been developed to approximate the necessary sample size to obtain the desired power. Choosing appropriate statistical procedure is a complicated task that demands specific knowledge and training. To determine an appropriate statistical procedure, it is important to keep in mind the following characteristics of the study: (1) purpose of the study (description, investigation of differences or associations); (2) type of sample (dependent or independent samples); (3) number of groups (one, two, or more); (4) level of measurement (nominal, ordinal, interval or ratio); and (5) distribution of the data (normal or not).