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