1. Experiments: Part 2

2. Overview
Review of some of the more advanced concepts in basic experimental design
Validities
Especially threats to internal validity
Advanced concepts
Matching
Counterbalancing
Block randomization

3. Validity in Experiments
Measurement validity
Statistical conclusion validity
External validity
Internal validity

4. Threats to Internal Validity
Something other than the experimental manipulation differs between groups and drives observed differences in the dependent variable
History threat vs. maturation threat
Testing threat vs. instrumentation threat
Regression toward the mean
Mortality (attrition, dropout)

5. Regression toward the Mean
Non-experiment: Pre-post within-subject design

6. Experimental design

7. Mortality

9. Selection Bias
87% % % %
Mortality

10. Advanced Concepts Matching Counterbalancing Block randomization
What is it? Assigning participants evenly across conditions based on some participant variable
Why is it used? Prevent “imbalance,” and thus confounding
Counterbalancing
What is it? Using varying sequences in within-subject designs
Why is it used? Prevent ordering effects
Block randomization
What is it? Rule that each condition must be completed before the next repetition
Why is it used? Keep sample sizes similar across conditions, increasing power (ability to detect a significant effect)