Independent variables are conditions varied or altered by the experimenter. The size, amount, or value of an independent variable is set by the experimenter. Independent variables are suspected causes of differences in behavior. The amount is controlled by the experimenter and it is suspected to be the cause of deterioration of driving ability. Dependent variables measure the results of the experiment.
Dependent variables reveal the effects that independent variables have on behavior.
If alcohol does affect driving ability, then subjects given alcohol should perform more poorly than subjects who do not receive alcohol. Extraneous variables are conditions the researcher wants to prevent from affecting the outcome of the study. If the experimenter didn't know this, he might conclude that there is no difference between people who drink alcohol and people who do not on a test of driving performance. Without a control group, it is impossible to know if the independent variable has an effect. Random Assignment of subjects to experimental and control conditions assures that the groups will be roughly equivalent before the independent variable is introduced.