Experiments: Part 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

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

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)

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

Experimental design

Mortality

Selection Bias 87% 84% 56% 77% Mortality

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)