This article discusses randomized clinical trials, double blind designs and factorial designs and describes and explains their main requirements. It is shown that the randomized clinical trial is not a different experimental design, but is one that reflects the purpose for and the scale on which the post-test only or pretest/post-test designs are often used. A common method for the prevention of experimenter bias in clinical trials is the double blind trial. Factorial designs make the simultaneous manipulation of two or more independent variables possible, thus producing data about their effect on the dependent variable.