Introduction to Research Design. Threats to Internal Validity One Group Pretest-Posttest Design. O X O. Campbell & Stanley “pre-experimental.” Wuensch “experimental.” OK design if can achieve “experimental isolation,” as in the chemistry lab.
Threats to Internal Validity
One Group Pretest-Posttest Design
Events other than X between pretest and posttest
Processes that normally cause subjects to change across time.
Pretesting subjects can change them.
The measuring instrument changes across time.
When scores have an error component, both high and low scores regress towards the mean upon retesting.
Subjects who drop out of the experiment may differ from those who stay in.