Ch.+10+ +Experimental+Design

8/13/2019 Ch.+10+ +Experimental+Design

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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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5

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30

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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

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