In this video, we will discuss principles of experimental design and learn some experimental design terminology. The four principles of experimental design are control, randomize, replicate, and block. To control,means to compare treatment of interest to a control group. To randomize, means randomly assigning subjects to treatments. To replicate, means collecting a sufficiently large sample within the study or to replicate the entire study. The last principle of experimental design is blocking. If there are variables that are know or suspected to effect the response variable, first group the subjects into blocks based on these variables. And then randomized cases with in each block to treatment groups. Lets discuss blocking a bit more, we would like to design and experiment to investigate if energy gels make you run faster. The treatment group gets the energy gel, the control group does not get any energy gel. It is suspected that energy gels might effect pro and amateur athletes differently therefore we block for pro status. To do so, we divide our sample into pro and amateur athletes, and then, we randomly assign pro and amateur athletes to treatment and control groups, therefor, pro and amateur athletes are equally represented in the resulting treatment and control groups. This way, if we do find a difference in running speed between the treatment and control groups we will be able to attribute it to the treatment, the energy gel, and can be assured that the difference isn't due to pro status since both pro and amateur athletes were equally represented in the treatment and control groups. So, how do we tell the difference between a blocking variable and an explanatory variable? Explanatory variables also sometimes called factors, are conditions we can impose on our experimental units. Blocking variables, on the other hand, are characteristics that the experimental units come with, that we would like to control for. Blocking is basically like stratifying, expect used in experimental settings when randomly assigning as opposed to when sampling. To wrap up our discussion on experimental design, let's go over a few new terms. A placebo is fake treatment, often used as the control group for medical studies. The placebo effect is when experimental unit show improvement simply because they believe they're receiving a special treatment. Blinding is when experimental units do not know whether they are in the control or the treatment groups. And finally, a double-blind study is one where both the experimental units and the researchers do not know who is in the control and who is in the treatment group. These terms will be useful as we examine more studies throughout the course.