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.

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.

The effect of X on Y depends on Z.

May be confounded with the independent variable when a single group is studied over an extended period of time.

This design eliminates the possibility of testing-X interaction.

An experiment in which two or more variables are manipulated.

A matter of how well an experiment controls for the effects of extraneous variables.

A threat to internal validity when a single group is measured at two or more points in time.

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.