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Lecture 14 Psyc 300A

Lecture 14 Psyc 300A. Review. Operational definitions Internal validity Threats to internal validity Type I and type II errors. Review: Operational definitions. For each of these studies, operationalize the IV and DV: The effect of exposure to other racial groups on prejudicial attitudes

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Lecture 14 Psyc 300A

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  1. Lecture 14Psyc 300A

  2. Review • Operational definitions • Internal validity • Threats to internal validity • Type I and type II errors

  3. Review: Operational definitions • For each of these studies, operationalize the IV and DV: • The effect of exposure to other racial groups on prejudicial attitudes • The effect of word length on speed of reading • The effect of cognitive therapy on depression

  4. Review: Internal validity • What is internal validity? • Ability to make valid inferences concerning the relationship between the IV and DV in an experiment. (effect on the DV is caused only by the IV) • The extent to which the results of an experiment can be attributed t the manipulation of the IV rather than to some confounding variable

  5. Review: Threats to Internal Validity • Nonequivalent control group • History • Maturation • Testing • Regression to the mean • Instrumentation • Mortality/Attrition

  6. Type I and Type II Errors

  7. Power • Power is the probability of avoiding a Type II error. (Finding an effect if there really is one there to find) • Power is related to: • Alpha level • Effect size (mean and sd) • Number of participants

  8. Review: Advantages of Multi-level Designs • What is a multi-level design? • Advantages: • Efficiency (fewer participants needed and less time) • Ability to see relationships better

  9. Review: Multifactor Designs • Factorial design: A design in which all levels of each IV are combined with all levels of the other IVs. • Advantages of factorial designs: • More efficient (fewer participants and less experimenter time) • Allows us to see how variables interact, see complex relationships

  10. Example: Movie Preferences

  11. What a Factorial Design Tells You • Main effect: The effect of an IV on the DV, ignoring all other factors in the study. (Compare means of different levels of IV, while ignoring [collapsing across] other IVs [ i.e., compare marginal means]) • Interaction effect: When the effect of one IV on a DV differs depending on the level of a second IV. • Interpret the interaction first

  12. Examples of Main Effects and Interactions • A1= morning • A2= late afternoon • B1= high fat diet • B2= low fat diet • DV: 0-50 rating of energy level

  13. More Main Effects and Interactions • A1= morning • A2= late afternoon • B1= high fat diet • B2= low fat diet • DV: 0-50 rating of energy level

  14. More Main Effects and Interactions • A1= morning • A2= late afternoon • B1= high fat diet • B2= low fat diet • DV: 0-50 rating of energy level

  15. Group Exercise: Main Effects and Interactions • Any questions from p.205 in book?

  16. Example: Psychotherapy Outcome

  17. Group Activity: Main Effects and Interactions Make graphs of the following situations:

  18. Factorial Designs: Naming Conventions • The first number is the number of levels in first IV, second number is number of levels in second IV, etc. • 2 x 2 • 2 x 3 • 2 x 2 x 3 • Between-subjects, repeated measures (within), mixed

  19. A 2 x 3 Interaction

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