An experimental study is a type of evaluation that seeks to determine whether a program or intervention had the intended causal effect on program participants. There are three key components of an experimental study design: (1) pre-post test design, (2) a treatment group and a control group, and (3) random assignment of study participants.
       A pre-post test design requires that you collect data on study participants’ level of performance before the intervention took place (pre-), and that you collect the same data on where study participants are after the intervention took place (post). This design is the best way to be sure that your intervention had a causal effect.
       To get the true effects of the program or intervention, it is necessary to have both a treatment group and a control group. As the name suggests, the treatment group receives the intervention. The control group, however, gets the business-as-usual conditions, meaning they only receive interventions that they would have gotten if they had not participated in the study. By having both a group that received the intervention and another group that did not, researchers control for the possibility that other factors not related to the intervention (e.g., students getting accustomed to a test, or simple maturation over the intervening time) are responsible for the difference between the pre-test and post-test results. It is also important that both the treatment group and the control group are of adequate size to be able to determine whether an effect took place or not. While the size of the sample ought to be determined by specific scientific methods, a general rule of thumb is that each group ought to have at least 30 participants.

       Finally, it is important to make sure that both the treatment group and the control group are statistically similar. While no two groups will ever be exactly alike, the best way to be sure that they are as close as possible is having a random assignment of the study participants into the treatment group and control group. By randomly assigning participants, you can be sure that any difference between the treatment group and control group is due to chance alone, and not by a selection bias.