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Chapter 10 Designing Quantitative Studies
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  1. Chapter 10Designing Quantitative Studies

  2. The Counterfactual Method • The counterfactual is what would have happened to the same people simultaneously exposed and not exposed to the causal factor. • Effect represents the difference between the two.

  3. Causality • The Counterfactual Method • Criteria for Causality—Lazarsfeld (1955) 1. Temporal 2. Empirical relationship 3. Relationship cannot be explained as being caused by a third variable

  4. Research Design Terminology in the Social Scientific and Medical Literature

  5. Experiments or Randomized Controlled Trial (RCT) Properties • Manipulation • Control • Randomization

  6. Manipulation • Doing something to study participants • Experimenter manipulates the independent variable by administering a treatment (intervention) to some subjects and withholding it from others, or by administering some other treatment

  7. Control Group • Researchers can expose the control group to various conditions: – no treatment – alternative treatment – placebo – standard treatment – different doses of the treatment – wait-list

  8. Randomization (Random Assignment, Random Allocation) • Involves placing subjects into treatment conditions at random • Approximates the ideal—but impossible—counterfactual of having the same people in multiple treatment groups simultaneously • Basic randomization

  9. Small Table of Random Digits

  10. Example of Random Assignment Procedure

  11. Breakdown of the Gender Composition of the Three Groups

  12. Experimental Designs • After-only (posttest-only) design • Before-after (pretest-posttest) design • Solomon four-group design • Factorial design • Randomized block design • Crossover (repeated measures) design

  13. Experimental Designs

  14. Symbolic Representation of a Pretest-Posttest Experimental Design R O1 X O2 R O1 O2 R = Randomization O = An observation or measurement X = An intervention

  15. Factorial Designs • Two or more variables are manipulated simultaneously • Test both main effects and interaction effects • Randomized block design • Crossover design

  16. Example of a Factorial Design

  17. Quasi-Experimental and Preexperimental Designs Nonequivalent control group pretest-posttest design (quasi-experimental)O1 X O2 O1 O2Nonequivalent control group posttest-only design (preexperimental) X O OOne group pretest-posttest design (preexperimental)O1 X O2

  18. Quasi-Experimental Designs • Time series design • Nonequivalent control group before-after design

  19. Time Series Design O1 O2 O3 O4 X O5 O6 O7 O8

  20. Other Quasi-Experimental Designs • Regression discontinuity design • Quasi-experimental dose-response analyses • Quasi-experimental (nonrandomized) arms of a PRPP randomization design

  21. Nonexperimental (or Observational) Research • Descriptive research • Correlational studies

  22. Designs of Correlational Studies • Retrospective (case-control) design • Prospective (cohort) designs • Natural experiments • Path analytic studies

  23. Continuum of Designs for Inferring Causality Strongest Weakest True experiment Quasi-experiment Pre-experiment Path analytic Prospective Retrospective Descriptive correlational correlational

  24. Descriptive Studies • Prevalence studies • Incidence studies