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