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The effect of X on Y depends on Z.
An experiment in which two or more variables are manipulated.
Pretests and posttests are administered to experimental conditions but random assignment is not possible.
This design eliminates the possibility of testing-X interaction.
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.
Loss of subjects varies across experimental conditions.
Controls for the effects of history in a trend analysis by comparing a group exposed to the treatment with other groups that are not.
May be confounded with the independent variable when a single group is studied over an extended period of time.
May occur when measurement of the dependent variable varies over time or experimental conditions.
A potential threat to validity when subjects are selected on the basis of their extreme scores on the dependent variable.