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:
- Selection
- History
- Maturation
- Repeated Testing
- Instrumentation
- Regression to the Mean
- Experimental Mortality
- Selection-Maturation Interaction
- Experimenter Bias
Before learning more about each one, first read the following
background information to a hypothetical experiment.
