Threats to Internal Validity
Between-groups Design
Participants
Group 1 Group 2
Experimental Group Control Group
Treatment
No Treatment
(Independent Variable)
Measurement Measurement
(Dependent Variable) (Dependent Variable)
Within-groups Design
(Repeated Measures)
Participants
Pretest
(Measure Dependent Variable)
Treatment
(Independent Variable)
Posttest
(Measure Dependent Variable)
Campbell & Stanley’s 8 Threats to
Internal Validity
1) _________
Groups _______ before the study began
Don’t allow _____-________
Don’t allow arbitrary assignment
Use ________ assignment
8 Threats to Internal Validity
2) Selection X __________
Experimental and control groups have
________ _______apart (developed
differently)
Difference is due to developmental change,
not ___________
8 Threats to Internal Validity
3) _________ Effects
Tendency for ________ scores to revert back
to ________ levels
Cover of Sports Illustrated Example
8 Threats to Internal Validity
4) ________
Groups differ due to
______ of participants
throughout study
8 Threats to Internal Validity
5) ___________
Change in participants due to _______
causes not treatment
Examples: physical, biological changes
(hungrier, older, bored)
8 Threats to Internal Validity
6) ________
Participants change due to _______ occurring
in outside world, environment
Example: 9/11
8 Threats to Internal Validity
7) ________
________ itself changes participants
“Practice Effect” – _________ effect caused
by previous experience with the dependent
measure
8 Threats to Internal Validity
8) ___________
Changes in ___________ can change
participant responses
Example: change in researchers,
measurement tool