ch.+10+ +experimental+design
TRANSCRIPT
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Chapter 10: Experimental Design
PSYC 3F40
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Assigning Participants (Ps) to Conditions
Three basic types of experimental designs:
randomized groups (between-person)designPs are assigned randomly to conditions
matched subjects designPs are matched intoblocks on the basis of a relevant variable (one thatcorrelates with the DV), then randomly assigned fromblocks to conditions
repeated measures (within-person) designeach P serves in all experimental conditions
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Posttest and Pretest-Posttest
Designs
Posttest only designdependent variable is
measured only after the manipulation of the
independent variable
Pretest-posttest designdependent variable is
measured twice, both before and after the
experimental manipulation
(repeated measures design)
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Advantages of
Pretest-Posttest Designs
Can determine that the experimentalconditions did not differ on the dependentvariable at the beginning of the experiment
Can see how much the independent variablechanged behavior from pretest to posttest
More powerful than posttest-only designs Each participant serves as his/her own control Removes error variance due to individual
differences
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Disadvantage of Pretest-Posttest
Designs
Pretest sensitizationadministering thepretest may lead participants to responddifferently to the independent variable than
they would had they not been pretested
Pretest-posttest designs are not essential.Posttest-only designs are adequate to
determine whether the independent variablehas an effect on the dependent variable.
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One-Way & Factorial Designs
A one-wayexperimental design involvesmanipulating only one independent variable(with at least two levels).
A factorial designinvolves manipulating two or
more independent variables (referred to as
factors).
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Describing the Size and Structure of
Factorial Designs
A 2 x 2 factorial (read 2-by-2) is a design withtwo independent variables, each with two levels.
A 3 x 3 factorial has two independent variables,each with three levels.
A 2 x 2 x 4 factorial has three independent
variables, two with two levels, and one with fourlevels.
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Assigning Participants to Conditions in a
Factorial Design (1)
Same options as with a one-way design:
Randomized groups factorial design
Matched groups factorial design
Repeated measures (within-person)factorial design
Also: Mixed factorial designparticipants
are randomly assigned to only one level ofsome independent variable(s) but receiveevery level of other independent variable(s);also called a between-within design
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Main Effects and Interactions
Main effects: The effect of an independent variable while ignoring the
effects of all other independent variables in the design.
A factorial design will have as many main effects as there
are independent variables. Interaction:
When the effect of one independent variable differs
across the levels of another independent variable.
E.g., the effect of variable A is different under one level ofvariable B than it is under another level of variable B.
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Graph of an Interaction
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Condition A1 Condition A2
Depe
ndentVariable
Independent Variable A
Condition B1
Condition B2
Variable A had a different effect on participants
in Condition B1 than on those in Condition B2.
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Higher-Order Designs
Three-way designs examine: the main effects of three independent variables
three two-way interactionsthe A X B interaction (ignoring
C), the A X C interaction (ignoring B), the B X C interaction(ignoring A).
The three-wayinteraction of A X B X C
The interaction between two variables depends on the level of theanother variable.
E.g., The A X B interaction is stronger in condition C1 than C2
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Example of a 3-way interaction
Ersner,Hershfield, Mikils, Sullivan, & Carstenson(2008, Study 1) examined time perspective and
mixed emotions
Perceiving that one has limited time left leads to:
The anticipation of loss A focus on emotionally meaningful goals
These two tendencies may yield complex, mixed
emotional experiences (e.g., positive and negative
emotions co-occuring)
Does time perspective cause mixed emotional
experience? Is this moderated by age?
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3 x 2 x 2 Mixed factorial design
3 guided imagery trials (within-person or repeated-measuresfactor): 1) Imagine a specific, meaningful location
2) Imagine the location in 2 months
3) Imagine the location in 4 months
2 conditions (between-person factor): 1) Control condition
2) Experimental condition: Imagine the location in 4 months, visiting itfor the last time
2 ages (between-person quasi-independent variable): 60 younger (M= 20 years) Ps
60 older (M= 77 years) Ps
DV: Mixed emotions = MINIMUM[Happiness, Sadness]
Example of a 3-way interaction
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2-way interaction
Time x Condition interaction, F(2, 115) = 12.33,p