Then, after manipulating the independent variable you will be better able to attribute the difference observed at the posttest to the independent variable because one group got a treatment and the other group did not. You want the groups to be the same on all other variables. Now we will discuss six techniques that are used to control for confounding variables. Random Assignment Random assignment is the most important technique that can be used to control confounding variables because it has the ability to control for both known and unknown confounding extraneous variables. Because of this characteristic, you should randomly assign whenever and wherever possible.
Random assignment makes the groups similar on all variables at the start of the experiment.
If random assignment is successful, the groups will be mirror images of each other. Ideally, research participants should be randomly selected from a population and then the selected individuals should be randomly assigned to the comparison groups. Because of the goal of establishing firm evidence of cause and effect in experimental research, random assignment is more important than random selection. Random assignment controls for the differential influence which confounding extraneous variables may have on a research study by insuring that each participant has an equal chance of being assigned to each comparison group. In other words, random assignment eliminates the problem of differential influence by making the groups similar on all extraneous variables.