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