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Chapter Ten Experimental and Ex Post Facto Designs

Chapter Ten Experimental and Ex Post Facto Designs. Independent and Dependent Variables. Variable : any quality or characteristic in a research investigation that has two or more values. Cause-and-Effect Relationship : the extent to which one

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Chapter Ten Experimental and Ex Post Facto Designs

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  1. Chapter Ten Experimental and Ex Post Facto Designs

  2. Independent and Dependent Variables • Variable: any quality or characteristic in a research • investigation that has two or more values. • Cause-and-Effect Relationship: the extent to which one • variable (the cause) influences another variable (the effect). • Independent Variable: a variable that the researcher studies • as a possible cause of something else; the variable that • the researcher directly manipulates. • Dependent Variable: a variable that is potentially influenced • by the independent variable; a variable that is influenced • by and to some extent depends on the independent • variable.

  3. The Importance of Control • Internal Validity: the extent to which the design of a research • study and the data it yields allows the researcher to draw • accurate conclusions about cause-and-effect and other • relationships. Without internal validity in experimental • designs, the results are not interpretable. • Confounding Variables: account for differences in two or • more groups that are not attributable to the particular • treatment or intervention being studied.

  4. Strategies for Controlling Confounding Variables • Keep some things constant. • Include a control group. • Randomly assign people to groups. • Assess equivalence before the treatment with one or • more pretests. • 5. Expose participants to all experimental conditions. • 6. Statistically control for confounding variables.

  5. Categories of Experimental Designs • Pre-Experimental Designs • True Experimental Designs • Quasi-Experimental Designs • Ex Post Facto Designs • Factorial Designs

  6. Pre-Experimental Designs • Not possible to show cause-and-effect relationships because • (a) the independent variable doesn’t vary or (b) experimental and • control groups are not comprised of equivalent or randomly • selected individuals. • Design 1: One-Shot Experimental Case Study (low internal validity) • Group 1 Tx Obs • Design 2: One-Group Pretest-Posttest Design • Group 1 Obs Tx Obs • Design 3: Static Group Comparison • Group 1 Tx Obs • Group 2 --- Obs

  7. True Experimental Designs • Compared to pre-experimental designs, experimental designs offer a • great degree of control and greater internal validity. • Design 4: Pretest-Posttest Control Group Design (random assignment) • Group 1 Obs Tx Obs • Group 2 Obs --- Obs • Design 5: Solomon Four-Group Design (enhances external validity) • Group 1 Obs Tx Obs • Group 2 Obs --- Obs • Group 3 --- Tx Obs • Group 4 --- --- Obs • Design 6: Within-Subjects Design (repeated measures) • Group 1 Txa Obsa • Txb Obsb R A N D O M R A N D O M

  8. Quasi-Experimental Designs • Randomness is not possible or practical; can’t control for all confounding • variables. • Design 8: Nonrandomized Control Group Pretest-Posttest Design • Group 1 Obs Tx Obs • Group 2 Obs --- Obs • Design 9: Simple Time-Series Design • Group 1 Obs Obs Obs Obs Tx Obs Obs Obs Obs • Design 10: Control Group, Time-Series Design • Group 1 Obs Obs Obs Obs Tx Obs Obs Obs Obs • Group 2 Obs Obs Obs Obs --- Obs Obs Obs Obs

  9. Quasi-Experimental Designs (con’t) • Design 11: Reversal Time-Series Design • Group 1 Tx Obs --- Obs Tx Obs --- Obs • Design 12: Alternating Treatment Design • Group 1 Txa Obs --- Obs Txb Obs --- Obs Txa Obs --- Obs Txb Obs • Design 13: Multiple Baseline Design

  10. Ex Post Facto Designs • The researcher identifies events that have already occurred or • conditions that are already present and then collects data to • investigate a possible relationship between these factors and • subsequent characteristics or behaviors. • - like correlational research, ex post facto research involves looking at • existing circumstances; • - like experimental research, ex post facto research has clearly • identifiable independent and dependent variables; • - unlike experimental research, ex post facto research involves no direct • manipulation of the independent variable – the presumed “cause” has • already occurred. • Design 14: Simple Ex Post Facto Design • Prior event Investigation period • Group 1 exp Obs • Group 2 ---- Obs

  11. Factorial Designs • Examination of the effects of two or more independent variables in a • single study. • Design 15: Randomized Two-Factor Design • Treatment (var.1) Treatment (var. 2) • Group 1 Tx1 Tx2 Obs • Group 2 Tx1 ----- Obs • Group 3 ---- Tx2 Obs • Group 4 ---- ---- Obs • Design 16: Combined Experimental and Ex Post Facto Design • Group 1a Txa Obs • Group 1 Expa Random assign Group 1b Txb Obs • Group 2 Expb Random assign Group 2a Txa Obs • Group 2b Txb Obs

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