Lecture 14 Psyc 300A

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Lecture 14Psyc 300A
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Review

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

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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

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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

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Review Threats to Internal Validity

• Nonequivalent control group
• History
• Maturation
• Testing
• Regression to the mean
• Instrumentation
• Mortality/Attrition

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Type I and Type II Errors
Accept the Null Hypothesis Reject the Null Hypothesis

Null is really True (chance is responsible) Null is really True (chance is responsible) Correct Decision Type I Error
Null is really False (chance is not responsible) Null is really False (chance is not responsible) Type II Error Correct Decision
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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

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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

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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

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Example Movie Preferences
Men Women Mean
Romantic 3 6 4.5
Action 7 4 5.5
Mean 5 5
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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

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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

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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

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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

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Group Exercise Main Effects and Interactions

• Any questions from p.205 in book?

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Example Psychotherapy Outcome
Pre Post MarginalMean
Cognitive 20 10 15
No Tx 20 20 20
MarginalMean 20 15
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Group Activity Main Effects and Interactions
Var A Var B AxB interaction
p lt .05 n.s. p lt .05
p lt .05 p lt .05 p lt .05
n.s. p lt .05 n.s.
n.s. n.s. p lt .05

• Make graphs of the following situations

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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

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A 2 x 3 Interaction