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The effect of X on Y depends on Z.
Pretests and posttests are administered to experimental conditions but random assignment is not possible.
A matter of how well an experiment controls for the effects of extraneous variables.
A threat to internal validity when a single group is measured at two or more points in time.
May occur when measurement of the dependent variable varies over time or experimental conditions.
Controls for the effects of history in a trend analysis by comparing a group exposed to the treatment with other groups that are not.
Loss of subjects varies across experimental conditions.
This design eliminates the possibility of testing-X interaction.
An experiment in which two or more variables are manipulated.
Randomization rules out this threat to internal validity.
Resembles the one-group pretest-posttest design except multiple observations are made before and after the treatment.
May be confounded with the independent variable when a single group is studied over an extended period of time.