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Experimental Design

Experimental Design

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Experimental Design

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  1. Experimental Design

  2. True Experimental Designs • Random assignment • Two comparison groups • Controls threats to internal validity • Strongest evidence for cause-effect

  3. Quasi Experimental Designs • Lacks randomization or • Lacks control group • Accommodates for natural setting

  4. Group Assignment • Completely randomized (between subjects) • Randomized block design • Repeated measures design (within subjects)

  5. Selecting a Design • How many IV are being tested? • How many levels does each IV have, and are these levels experimental or control conditions? • How many groups of subjects are being tested?

  6. Selecting a Design • How will subjects be selected, and how will they be assigned to groups? • How often will observations of responses be made? • What is the temporal sequence of interventions and measurements?

  7. True Experimental Designs • Single factor designs for independent groups • One way design

  8. Design 1A • Pretest-Posttest control group design • R O1 X O2 • O1O2 • Standard in Clinical research for cause -effect • Randomized controlled trial (RCT)

  9. Design 1B • Two group pretest-posttest design • O1 X1O2 • R O1 X2O2 • New vs. old • Difference between 2 treatments

  10. Design 1C • Multigroup pretest-posttest control group design • O1 X1 O2 • R O1 X2 O2 • O1O2 • Threat to internal validity is attrition • Threat to external validity is the interaction of treatment and testing

  11. Analysis of Design 1 • Utilize change scores • Interval-ratio data 2 groups unpaired t-test • 3 groups ANOVA • Ordinal data 2 groups Mann-Whitney U • 3 groups Kruskal-Wallis analysis of variance

  12. Design 2 • Posttest-only design • R X1 O2 • O2 • True experimental design • Internal validity is strong • External validity is strong • Analysis is identical to Design 1

  13. Multifactor Designs for Independent Groups • Design 3 Factorial • Design 4 Randomized Block • Design 5 Nested

  14. Design 3 Factorial Design • 2 or more Independent variables • Has factor and levels • 2 x 2 • 3 x 3 • 4 x 4

  15. Two Way Factorial Design • Main effect • Interaction effect • Analysis is two way or three way analysis of variance

  16. Design 4: Randomized Block Design • Concern over an extraneous variable • Attributed variable is crossed with an active IV • Analysis is 2 way analysis of variance or multiple regression analysis

  17. Design 5: Nested Design • Some attribute variables cannot be crossed

  18. Repeated Measures Design • Design 6 One Way Repeated Measures Design • Design 7 Crossover Design • Design 8 Latin Square Design

  19. Design 6: One Way Repeated Measures Design • All subjects exposed to all levels of one treatment variable • X1 O1 • X2 O2 • X3 O3 • Internal threat is carry over • Analysis one-way ANOVA

  20. Design 7: Cross Over Design • Minimizes bias of test sequence • Counterbalances treatment conditions • Washout period • R O1 X1 O2 ... O3 X2 O4 • O1 X2 O2 ... O3 X1 O4 • Analysis: Paired T-test or 2 way ANOVA • Ordinal data Wilcoxon signed ranks test

  21. Design 8: Latin Square Design • Sequencing of treatment order is concern • Randomizes treatment order • 3 x 3 or 4 x 4

  22. Multifactor Designs for Repeated Measures • Design 9 Two-way design with two repeated measures • Design 10 Mixed design

  23. Design 9: Two-way Design with Two Repeated Measures • Analysis: Two way analysis of variance with two repeated measures to measure main and interaction effects

  24. Design 10: Mixed Design • Analysis: 2 way anova with one repeated measure for main effect two way design with one repeated factor for interaction effects

  25. Quasi Experimental Designs • Do not use random assignments • Potential bias of sample • Strong control over design

  26. Quasi Experimental Designs • Design 11 One-group pretest-posttest design • Design 12 Time series design • Design 13 Nonequivalent pretest-posttest control group design • Design 14 Nonequivalent posttest-only control group design

  27. Design 11: One-group Pretest-posttest Design • O1 X O2 • Threat to internal validity-no comparison group • Threat to external validity-potential interactions with selection • Analysis: Paired t-test for ratio/internal • Wilcoxon signed ranks test for ordinal

  28. Multigroup Design • Design 12 Time series design • O1O2O3O4 X O5O6 O7O8 • Effect of tx on physiological or psychological variables over time • IV is time • Threat to internal validity is history • Variations 2 or more groups, tx withdrawal

  29. Design 13: Nonequivalent Pretest-posttest Control Design • O1 X O2 O1O2 • Threat to internal validity is interaction of selection with history and maturation • Analysis: Unpairted T-test, anova-interval or ratio • Mann-Whitney U-test-Ordinal

  30. Design 14: Nonequivalent Posttest-Only Control Group Design • X O O • Threat to internal validity is selection bias and attrition • Analysis: Regression