Selection and differential attribution are the primary threats to internal validity of this design.

Pretests and posttests are administered to experimental conditions but random assignment is not possible.

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.

An experiment in which two or more variables are manipulated.

May occur when measurement of the dependent variable varies over time or experimental conditions.

This design eliminates the possibility of testing-X interaction.

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.

The effect of X on Y depends on Z.

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