Chapter 9: Experimental Design

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Chapter 9 Experimental Design

• One-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

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• 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.

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• 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).

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• 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

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• 3 x 3 design is a two way design with three
  levels of each independent variable
• 100 200
  300
• 0
• 5
• 10

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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

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

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• 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

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• 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.

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• 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.

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• 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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Memory for Words as Function of Word Length and
Caffeine Intake
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No interaction, the lines are parallel. The
effect of therapy is the same for the drug and no
drug group.
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A strong interaction, the effect at one level of
the independent variable is opposite of the other
level.
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• 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).

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• 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

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• 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.

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