Threats to Validity
I. Internal Validity
Extraneous variables vs. Confounding variables
Extraneous variable--any variable in a study other than the two variables of
interest
Confounding variable--an extraneous variable (usually unmonitored) that is
allowed to change systematically along with the two variables being
studied...prevents you from establishing a causal link between your two
variables of interest
Threats to Internal Validity (p. 161-170)
Category 1: For all studies:
Environmental variables
Category 2: For studies comparing different groups:
assignment bias
Category 3: For studies comparing one group over time:
history effects--environmental events other than the treatment that occur between the first treatment condition and the last treatment condition and may affect the results
maturation--systematic changes in physiology or psychology that occur during study and may affect the participants' scores
instrumentation--changes in a measuring instrument that take place during the course of the study
testing effects--practice effects, fatigue, carry-over effects
regression toward the mean
II. Threats to External Validity
Category 1: Generalizing across participants
1. Subject selection bias (cost-restrictive sampling)
2. College students
3. Volunteer bias
4. Participant characteristics
5. Cross-species generalizations
1. Novelty effect
2. Reactivity
3. Demand characteristics
4. Multiple treatment interference
Category 3: Generalizing across experiments
1. Experiment bias
2. Experimenter characteristics
Category 4: Generalizing across features of the measures
1. Sensitization
2. Generality across response measures
3. Time of measurement