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Pretests and posttests are administered to experimental conditions but random assignment is not possible.
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.
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.
Controls for the effects of history in a trend analysis by comparing a group exposed to the treatment with other groups that are not.
This design eliminates the possibility of testing-X interaction.
May occur when measurement of the dependent variable varies over time or experimental conditions.
A threat to internal validity when a single group is measured at two or more points in time.
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.
Selection and differential attribution are the primary threats to internal validity of this design.