Chapter 9: Experimental DesignOne-way design: Only one independent variable is

manipulated. Must have a minimum of two levels• Simplest is a two group experimental design

• Compare memory for words and nonwords

• Randomized groups: between groups design where participants are randomly assigned to conditions

• Matched-subjects: participants are matched on a variable related to the dependent variable and then randomly assigned to conditions

• Repeated measures: within-subjects design where each participant completes all conditions

• Posttest design: measure participants on the dependent variable only after the independent variable is manipulated.

• Pretest-Posttest design : Participants are first tested on the dependent variable, then the independent variable is manipulated, and then they are again tested in the dependent variable.• Ensures participants in each condition were initially

equivalent in the dependent variable.• If there is a pretest score, researchers can see how

much the dependent variable changed.• More powerful design, making it easier to detect

effects of the independent variable.

• Pretest sensitization: pretest may sensitize participants to respond to the independent variable differently than they would if they did not have the pretest.

• Examining effect of caffeine on memory. First pretest all participants on the memory test, then give one group caffeine and the other group a placebo, and then test their memory again.

• The memory pretest may affect their later performance on the memory test (may have practiced, are more familiar with the test).

Factorial Design• One-way design: test one independent variable• Factorial design: manipulate two or more

independent variables. • Factor: refers to each independent variable

• Two-way design: two independent variables• Three-way design: three independent variables:

2 x 2 design (two way) with 2 levels of each independent variable

Words Nonwords

Short

Long

3 x 3 design is a two way design with three levels of each independent variable

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2 x 2 x 3 is a three way design (three independent variables) with two and three levels each

A1: Words A2: Nonwords

B1 B2 B1 B2

C1 C1

C2 C2

C3 C3

2 x 3 is a two way design (two independent variables) but one variable has two levels and one variable has three levels

A1NoTherapy

A2CognitiveTherapy

A3BehaviorTherapy

B1No Drug

B2Drug

Assignment of participants to conditions:• Randomized groups factorial design: participants

are randomly assigned to one of the combinations of the independent variables

• Matched factorial design: match participants based on their score on a measure related to the dependent variable. • If there are 6 cells then choose six highest scores

and randomly assign each to a cell.• Repeated measure factorial design: all

participants complete all conditions (cells)• So in a 2 x 2 (4 cells) each participant completes

4 conditions, in a 3 x 3 x 2 (18 cells) each participant completes all 18 conditions

• Mixed factorial design: has at least one between and one within subjects variable.

• 2 x 2, one independent variable between subjects (caffeine no caffeine) and other independent variable is within subjects (visual memory test, verbal memory test)

• Randomly assign participants to the between subjects condition and all participants complete the within subjects condition.

Main Effect: when there is an effect (or difference) for one independent variable, collapsing across the other independent variable.

• 2 (caffeine, no caffeine) x 3 design (short, medium, long words). • It is a two way design, so there will be two main effects

(one for caffeine and one for word length)• If there is a main effect of caffeine, that means that

memory is better in individuals who have caffeine regardless of the length of words (average across words).

• If there is a main effect of word length it may be that memory is better for shorter than longer words, collapsing across caffeine intake.

Memory for Short, Medium, and Long Words as a Function on Caffeine Intake

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Short Medium Long

Word Length

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mb

er

of

Wo

rds

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em

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be

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Caffeine

No Caffeine

Interaction: Occurs when the influence of one independent variable is different at different levels of another independent variable.

• 2 (caffeine, no caffeine) x 2 design (short, long words)

• The effect of caffeine on memory may be different for short words than for long words. • Caffeine may only have a positive effect on memory

for short words, but there may be no difference between caffeine and no caffeine groups for long words.

• Caffeine interacted with word length

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of W

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caffeineno caffeine

Memory for Words as Function of Word Length and Caffeine Intake

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Therapy No Therapy

Dep

ress

ion

No DrugDrug

No interaction, the lines are parallel. The effect of therapy is the same for the drug and no drug group.

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VerbalVisual

A strong interaction, the effect at one level of the independent variable is opposite of the other level.

Higher-Order designs: • Three-way designs: 2 x 2 x 3

• There will be three main effects (A,B,C)

• There will be three two-way interactions: A by B (ignoring C), A by C (ignoring B), and B by C (ignoring A).

• There will be one three-way interaction (A by B by C).

Subject Variable: based on individual personal characteristics that you can not manipulate but you can measure (age, IQ, gender).

Expericorr (mixed) design: has at least one independent variable manipulated by the researcher and at least one subject variable (gender) measured but not manipulated.• Allows researchers to examine effects of a subject

variable• Help to understand how personal characteristics

relate to behavior (age, gender)• Dividing participants into groups based on a

subject variable makes the participants in each group more homogeneous

Classifying participants into groups: • Median-split procedure: divide participants into two

groups based on the median score (half of the participants above and half below this score).

• Extreme groups procedure: divide participants into two groups based on very high and low scores on a variable of interest.

• These procedures can throw away valuable information.

• With median split the participants may not truly be high or low, they may just be high or low in the sample but not in the population.

Cautions with mixed designs:

• The subject variable it not manipulated so this simply provide correlational findings, but not causality.