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Chapter 8 part 1(1)



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  • 1. Chapter 8
  • 2. Research Design The plan or strategy used to answer the research question This is where we put to use all of the things that we have learned about validity and reliability.
  • 3. Experimental Research Design• Weak vs. Strong experimental design
  • 4. Weak Experimental Designs Do not control for many extraneous variables Provide weak evidence of cause and effect Have threats to internal validity
  • 5. One-group Posttest- OnlyDesign Influence of a treatment is investigated with only one group of individuals  Single test after treatment (“posttest”) Example: New reading program in 1st grade  Does program improve reading skills?
  • 6. One-group Posttest-onlyDesign Treatment Posttest Measure X O
  • 7.  Problems? Extraneous variables? Why/when would you use this design? If there is background information on DV and prior work has shown mechanism by which IV impact DV.
  • 8. One-group Pretest-Posttest Design A treatment condition is interjected between pre- and posttest of the dependent variable.Pretest measure Treatment Posttest Measure O X O Compare
  • 9. One-group Pretest-Posttest Design Advantage over one-group posttest? Problems? Threats to internal validity? Why would we use this design?  If we cannot create an equal group for comparison, we can get SOME information this way
  • 10. Nonequivalent Posttest-Only Design  Performance of an experimental group is compared with that of a nonequivalent control group at posttest Posttest Treatment MeasureExperimental Group X O CompareControl Group O
  • 11.  Advantage over previous designs? Problems? Threat to internal validity?  Selection Best thing to do is to use random assignment.  Creates equivalent groups Next best is to use matching on relevant variables.  Will discuss this next chapter
  • 12. Strong Experimental ResearchDesigns Designs that effectively control extraneous variables and provide strong evidence of cause and effect
  • 13.  To control for extraneous variables you must eliminate potential rival hypotheses. Done in two ways:  Control Techniques- more on this next week (ch. 7)  Control Group
  • 14.  Control group  comparison group  Does not receive “active” level of IV ○ No intervention or standard experience Experimental group  receives a level of the IV intended to produce effect
  • 15. Functions of a Control Group1. Serve as a comparison to the experimental group. Did the treatment/manipulation have an effect?Estimate Counterfactual- what the participants’ responses would have been had they not received the treatment
  • 16. Functions of a Control Group2. Control for rival hypotheses  With random assignment, extraneous variables will have an equivalent impact on both groups
  • 17. Strong Experimental ResearchDesigns Basic designs – one IV and one DV  Between-participants  Within-Participants (repeated measures) Factorial Designs – multiple IVs
  • 18. Posttest-Only Control Group Design  This design looks familiar, right?  What is different now? Posttest Treatment MeasureExperimental Group X O CompareControl Group O
  • 19. Posttest-Only Control GroupDesign We could have more than 1 experimental group Posttest Treatment MeasureControl Group OExperimental Group 1 X1 O CompareExperimental Group 2 X2 O
  • 20. Important time-out: Benefits of randomized control group Avoid many threats to internal validity  Examples?
  • 21. Pretest-Posttest Control GroupDesign Simply add pretest to previous design What comparisons will we make? Pretest Posttest Treatment Measure Measure Experimental O X O Group Control O O Group
  • 22. Benefits of Pretest Ensure equivalency of groups Detect ceiling and floor effects  Select participants accordingly Can empirically demonstrate effect of treatment  Change scores See if initial position on DV is important  Treatment may affect hi/low differently