Experimental psychologists select or manipulate one or more conditions in order to determine their effects on a measure of the behavior of a subject. For example, the smell of delicious food may be presented periodically to subjects in order to assess its effect on their salivation response.
The manipulated condition is referred to as the independent variable, and the behavioral measure is called the dependent variable. In our example, the smell of delicious food would be the independent variable and the salivation response would be the dependent variable.
An experiment is internally valid to the extent that it shows a cause-effect relationship between the independent and dependent variables. Suppose the experimenter observes an increase in the probability of salivating after presenting the smell of delicious food, leading him or her to conclude that the food smell produces salivation. For this conclusion to be internally valid, the experiment must be designed so that conditions other than the food smell are ruled out as potential causes for the behavior change. For example, if the sight of the food is presented along with its smell, then an alternative explanation could be that the sight of the delicious food, and not its smell, is responsible for the increased probability of salivating.
There are nine sources of threat to internal validity. They are:
Before learning more about each one, click Next to read the following background information to a hypothetical experiment.