1. Lecture 14 Psyc 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 Accept the Null Hypothesis Reject the Null Hypothesis Null is really True (chance is responsible) Correct Decision Type I Error Null is really False (chance is not responsible) Type II Error Correct Decision

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 MenWomenMean Romantic364.5 Action745.5 Mean55

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 PrePostMarginal Mean Cognitive201015 No Tx20 Marginal Mean 2015

17. Group Activity: Main Effects and Interactions Make graphs of the following situations: Var AVar BAxB interaction p <.05 n.s. p <.05 n.s.p <.05 n.s. p <.05

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