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Pretests and posttests are administered to experimental conditions but random assignment is not possible.
Controls for the effects of history in a trend analysis by comparing a group exposed to the treatment with other groups that are not.
The effect of X on Y depends on Z.
This design eliminates the possibility of testing-X interaction.
May occur when measurement of the dependent variable varies over time or experimental conditions.
Loss of subjects varies across experimental conditions.
A matter of how well an experiment controls for the effects of extraneous variables.
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.
An experiment in which two or more variables are manipulated.
Selection and differential attribution are the primary threats to internal validity of this design.