How does random assignment accomplish the goal of controlling for the influenced of extraneous or confounding variables?
Random assignment starts with a group of research participants. Then using the process of random assignment, these participants are randomly assigned to two or more groups. Random assignment means that the researcher is taken out of the loop of making decisions about who goes into the different groups. Instead, the mathematical theory of probability is used to conduct random assignment. Because random assignment is so important, here is a little more about it…Random assignment equates the groups on all known and unknown extraneous variables at the start of the experiment. This makes it very plausible that any significant observed difference between the groups on the DV after the administration of the IV can be attributed to the effect of the IV. Even when you use another control technique (e.g., matching or analysis of covariance) the experimental research design is dramatically improved if random assignment is also used. Random assignment is what I (Johnson) call the secret ingredie
