this case, the operational definition of driving skills is in milliseconds of stopping time. Another operational definition of driving skills would be how many times the person playing the driving game crashes the virtual car. Often, a single depen- dent variable can have many operational definitions. Clear operational definitions enable other researchers to know how you measured your variables. Knowing your methods, other researchers can use identical methods in gathering data that will build on your data, supporting or differing from your results.
GROUpS Your experiment tests the hypothesis that the dependent vari- able (driving skills, as measured by either the number of virtual crashes or how long it takes the driver to stop the car) is affected by the independent variable (the amount of alcohol consumed, as shown by BAC). But how do you know if the change in the dependent variable is really caused by your manipulation of the independent variable?
To clarify the relationship between your variables, you assign some participants in the study to a control group. In this case, participants assigned to the control group drink something nonalcoholic, such as tonic water, instead of alcohol. To make sure that both groups expect to be drinking alcohol, you might disguise the tonic water by using lemon or even some alcohol rubbed along the rim of the glass. However, the participants in the experimental group experience the manipulation you are interested in. In this case, they consume enough alcohol to reach a certain BAC level. Because you disguise the tonic water, all the study participants think they are drinking alcohol. You then compare the effects of drinking tonic water in the control group with the effects of drinking alcohol in the experimental group (or more than one experimental group, if you include several levels of BAC in the study).
The benefit of using an experimental method is that the researcher can study the causal relationship between the two variables. Suppose the independent vari- able (BAC in our example) consistently influences the dependent variable (driv- ing skills). The independent variable is then assumed to cause the change in the dependent variable. The experiment makes it possible to rule out alternative explanations, such as third variables. In addition, because the experimenter deter- mines when the independent variable (here, the BAC level) is administered, the experimenter can be sure that this variable comes first in time. In this way, the experimenter also solves the directionality problem.
CONTROL IS NECESSaRY TO DETERMINE CaUSaLITY A properly perfor- med experiment depends on rigorous control. Here, control means the steps taken by the researcher to minimize the possibility that anything other than the indepen- dent variable will affect the experiment’s outcome. When conducting an experi- ment, a researcher needs to ensure that the only thing that varies is the independent variable. That way, the researcher knows that the independent variable— nothing else—has affected the dependent variable. Anything that affects a dependent