1. Internal Validity

2. All about whether the research design (and data analysis) warrants the conclusions. Concerned with: – Causal relationships – Various threats to internal validity Single group Multiple group Social effects Experimenter effects

3. Causal Relationships Three conditions are needed to establish a casual relationship: – Temporal condition: Presumptive cause must precede the effect. – Statistical relationship between cause and effect. Correlation t test, ANOVA, or other statistical test of a null hyp. Elimination of plausible alternative explanations. – This is usually the most difficult to esbablished.

4. Plausible Alternative Explanations Single group threats Posttest only design:X--------------->O or Prettest-postest design:O-------X------>O Note X is a treatment O is an observation or measurement Threats include: History, maturation, testing, instrumentation, mortality, regression (to the mean)

5. Plausible Alternative Explanations Multiple group threats Typically involve interactions of single-group threats and selection (of participants). Selection-history Selection-maturation Selection-testing Selection-instrumentation Selection-regression

6. Social Effects Effects that result from social interaction Diffusion of treatment Imitation of treatment Compensatory Rivalry (John Henry Effect) Resentful Demoralization