Random Assignment

The use of a random procedure to place research participants into the various groups or conditions of a study. By using a random procedure, each participant has an equal chance of being assigned to any one of the groups or conditions.

The purpose of random assignment is to increase the probability that the groups or conditions will be equivalent, on average, at the start of the study with respect to all extraneous variables that could affect the phenomenon being studied (that is, the dependent variable). In this way, any differences among the groups or conditions at the end of the study can be attributed to the effects of the variable being tested for its causal effects (that is, the independent variable).