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CA (650,000)
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PSYB01H3 (200)
Anna Nagy (100)
Lecture 7

PSYB01H3 Lecture Notes - Lecture 7: Demand Characteristics, Habituation, Covariance


Department
Psychology
Course Code
PSYB01H3
Professor
Anna Nagy
Lecture
7

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PSYB01 - Lecture 7
CHAPTER 8
EXPERIMENTAL DESIGN
LEARNING OBJECTIVES
Describe the relationship between a confounding variable and the internal validity of an experiment
Explain error variance
Describe the post-test only design and the pretest-posttest design, including the advantages and
disadvantages of each design
LEARNING OBJECTIVES
Contrast an independent groups design with a repeated measures design
Summarize the advantages and disadvantages of using a repeated measures design
Describe a matched pairs design, and state reasons why one might use this design
CONFOUNDING AND INTERNAL VALIDITY
Internal Validity: experiment must be designed and conducted so that only the independent variable can
be the cause of the results
TWO WAYS TO MANIPULATE INDEPENDENT VARIABLE
1. Vary it quantitatively
2. Vary it qualitatively
CONFOUNDING AND INTERNAL VALIDITY
Confounding Variable: confounding occurs when the effects of the independent variable and an
uncontrolled variable are intertwined so one cannot determine which is responsible for the effect
CONFOUNDING VARIABLES
Tom Rogers wanted to test a new "singalong" method to teach math to fourth graders (e.g., "I love to
multiply" to the tune of "God Bless America"). He used the singalong method in his first period class. His
sixth period students continued solving math problems with the old method. At the end of the term, Mr.
Rogers found that the first period class scored significantly lower than the sixth period class on a
mathematics achievement test. He concluded that the singalong method was a total failure.
1. Identify the independent variable(s).
2. Identify the dependent variable(s).
3. Identify any confounding variable(s).
4. Propose a method to "unconfound" the experiment.
ERROR VARIANCE
“statistical variability of scores caused by the influence of variables other than your independent
variables (extraneous or subject related variables)” (Bordens & Abbott, 2005)
Rarely possible to hold all variable constant
Subjects differ from one another
Environmental conditions fluctuate
Subjects change over time
May cause fluctuation in scores that have nothing to do with your IV (produce error variance)
BASIC EXPERIMENTS
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Posttest-Only Design
o Must:
Obtain two equivalent groups of participants
- Selection differences
Introduce the independent variable
Measure the effect of the independent variable on the dependent variable
POSTTEST-ONLY DESIGN
POST-TEST ONLYHYPOTHETICAL EXPERIMENT
Performance on Dependent Measure (Simulated Air Control Task)
Control Group Experimental Group (THC)
25 13
24 19
18 22
29 18
19 23
Mean 23 Mean 19
REDUCING ERROR VARIANCE
Hold extraneous variables constant (treat participants exactly the same except for IV)
Random Assignment
Larger sample?
Match participants?
Carefully select levels of IV
Statistical Analysis (estimate probability that error variance would produce observed results)
BASIC EXPERIMENTS
Pretest-Posttest Design
A pretest is given before the experimental manipulation is introduced to make sure groups are
equivalent at the beginning of the experiment
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