Experiments: Part 3

Overview How do the main types of validity apply to experimental research? What are some of the main threats to internal validity? What confusing concepts would you like to review before Exam 2?

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 Imbalance 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