Experiments

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Experiments

  1. 1. Experiments, Quasi-Experiments, & Natural Experiments causal inference and experimentation designs and validity natural experiments
  2. 2. randomized controlled experiment Treatment or R experimental group Population Sample R Control Group R = randomization
  3. 3. Randomized Controlled Experiment Pre-test Post-test R Experimental Group Mexp1 x Mexp2 R Control Group Mcontrol1 Mcontrol2 X = experimental manipulation M = measurements Experimental effect = (Mexp2 - Mexp1)- (Mcontrol2 - Mcontrol1) R = Random assignment of subjects to groups
  4. 4. designs and validity • last time: validity of measures and indicators • this time: internal & external validity as aspects of research design
  5. 5. internal validity XZ • extent to which a research design yields strong evidence of causality • Key issue: is there a causal relationship between a “treatment” and the “outcome(s)”? Threats to internal validity: 1. History 2. Maturation 3. Testing 4. Instrumentation 5. Statistical Regression 6. Selection Threat 7. Experimental Mortality 8. Selection-Maturation Interaction
  6. 6. external validity XZ • extent to which a research design yields findings that generalize to • To subjects / objects / units other than those actually in your study • To settings / circumstances other than that/those actually in your study • To times other than those actually in your study Threats to external validity: 1. Interaction of testing and experimental variable 2. Interaction of selection bias and experimental variable 3. Reactive Arrangements 4. Multiple-treatment interference
  7. 7. How might you design an experiment to test a new drug?
  8. 8. How might you design an experiment to test a new drug?
  9. 9. OXO 000 X 000 00 X 00 X 00 X 00
  10. 10. Dosage and Factorial Designs
  11. 11. regression effect: matching and Head Start
  12. 12. Natural Experiments
  13. 13. Posner • Why are the Chekwas and Tumbukas allies in Zambia and adversaries in Malawi? • unit of analysis (530) • case selection criteria (531) • survey design (532) • lit review (535) • additional tests (541) • conclusions (543) • Posner’s approach: find the treatment
  14. 14. Dunning: Important questions to ask of natural experiments
  15. 15. group activity 1. Characterize your project, or a partner’s project, in terms of its research design 2. What are some of the potential threats to internal and external validity?

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