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CHAPTER 8

CHAPTER 8. EXPERIMENTAL DESIGN. LEARNING OBJECTIVES. Describe the relationship between a confounding variable and the internal validity of an experiment Describe the posttest-only design and the pretest-posttest design, including the advantages and disadvantages of each design.

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CHAPTER 8

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  1. CHAPTER 8 EXPERIMENTAL DESIGN

  2. LEARNING OBJECTIVES • Describe the relationship between a confounding variable and the internal validity of an experiment • Describe the posttest-only design and the pretest-posttest design, including the advantages and disadvantages of each design

  3. LEARNING OBJECTIVES • Contrast and compare independent groups design with a repeated measures design • Summarize the advantages and disadvantages of using a repeated measures design • Describe a matched pairs design, including reasons to use this design

  4. CONFOUNDING AND INTERNAL VALIDITY • Confounding Variable • Another variable that occurs along with the independent variable • Is an uncontrolled variable • Cannot determine which variable is responsible for the effect

  5. CONFOUNDING AND INTERNAL VALIDITY • Internal Validity • An experiment must be designed and conducted so that only the independent variable can be cause of the results • Internal validity exists when the results of an experiment can be confidently attributed to the effect of the independent variable

  6. BASIC EXPERIMENTS • Posttest-Only Design • Must: • Obtain two equivalent groups of participants • Care must be taken to eliminate selection differences • Introduce the independent variable • Measure the effect of the independent variable on the dependent variable

  7. POSTTEST-ONLY DESIGN

  8. BASIC EXPERIMENTS • Pretest-Posttest Design • A pretest is given to each group prior to introduction of the experimental manipulation • Assures that groups are equivalent at the beginning of the experiment • Can quickly measure changes that occur from the pretest to the posttest

  9. BASIC EXPERIMENTS • Advantages of the pretest-posttest design • Mortality (dropout factor) • Assess equivalency of groups with small sample size • Can be used to select participants for the experiment

  10. BASIC EXPERIMENTS • Disadvantages of the pretest-posttest design • Time consuming and awkward to administer • Sensitizes participants to what is being studied • Demand characteristics • Reduces external validity

  11. ASSIGNING PARTICIPANTS EXPERIMENTAL CONDITIONS • Independent Groups Design • Participants participate in only one group • Repeated Measures Design • Participants are in all conditions

  12. REPEATED MEASURES DESIGN • Advantages and Disadvantages of Repeated Measures Design • Advantages • Fewer participants • Extremely sensitive to statistical differences • Conditions are identical because person is own control group • Disadvantages • Order effect • Practice effect • Fatigue effect • Carryover effect

  13. REPEATED MEASURES DESIGN • Counterbalancing • Complete counterbalancing • Latin squares • Time Interval Between Treatments • Choosing Between Independent Groups and Repeated Measures Design

  14. COUNTERBALANCING

  15. LATIN SQUARE WITH FOUR CONDITIONS

  16. MATCHED PAIRS DESIGN • Goal is to Match People on a Participant Characteristic • Matched to either the dependent measure or a variable that is strongly related to the dependent variable

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