Lecture 14Psyc 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– The effect of word length on speed of

reading– The effect of cognitive therapy on

depression

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

Review: Threats to Internal Validity

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

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

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

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

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

Example: Movie Preferences

0

2

4

6

8

10

Romantic Action

Movie Type

Ra

ting

men

women

Men Women Mean

Romantic

3 6 4.5

Action 7 4 5.5

Mean 5 5

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

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

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

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

Group Exercise: Main Effects and Interactions

• Any questions from p.205 in book?

Example: Psychotherapy Outcome

01

02

0

Pre PostTime

BD

I Sco

re

Cognitve

No Tx

Pre Post MarginalMean

Cognitive

20 10 15

No Tx 20 20 20

MarginalMean

20 15

Group Activity: Main Effects and Interactions

Make graphs of the following situations:

Var A Var B AxB interaction

p < .05 n.s. p < .05

p < .05 p < .05 p < .05

n.s. p < .05 n.s.

n.s. n.s. p < .05

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

A 2 x 3 Interaction

0

2

4

6

8

10

Relaxation Focus Affirmation

Coping Technique

Rat

ing hi stress

lo stress