BetweenSubjects Design

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Between-Subjects Design

• Psych 7
• February 20th

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Announcements

• Midterm Information
• Mean 72
• Scores will not be curved (if need be) until the
  END OF THE QUARTER
• Multiple Choice see Dina
• Extra OHs on Friday from 230 - 430
• Short Answer see me
• Regular OHS on Wednesday from 130 330

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Outline

• Between-Subjects Designs
• Characteristics
• Advantages and disadvantages
• Individual differences as confounding variables
• Individual differences and variability
• Other threats to internal validity

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Between- vs. Within-Subjects Designs

• Determine causal relationship between 2 variables
  by comparing different groups of scores
• Two Basic Research Designs
• Between-Subjects Design
• Within-Subjects Design

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Characteristics of Between Subjects Designs

• Compares separate groups of individuals
• Separate group of Ps assigned to each level of IV
• Only one score per participant
• Also referred to as an independent-measures
  design
• Goal is to determine whether differences exist
  between 2 treatment conditions

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Bargh, Chen, and Burrows (1996)

• Goal Examine the influence of stereotypic primes
  on behavior
• Ps placed in one of two conditions
• C1 elderly prime
• C2 neutral prime
• Measured how quickly Ps walked down the hall
  after being primed

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Bargh, Chen, and Burrows (1996) Results
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Advantages and Disadvantages

• Advantages
• Not influenced by practice effects, fatigue or
  boredom, or contrast effects
• Disadvantages

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Example of Individual Differences

• John
• John is a 21-year old White male. He is 510
  tall, weighs 180 pounds, and has blue eyes, blond
  hair, and an IQ of 110. He comes from a middle
  class family with one older sister. John is a
  chemistry major and was awake until 2am this
  morning after celebrating his success on a chem
  exam. He comes to this experiment with only 4
  hours of sleep, suffering from a mild hangover.

• Mary
• Mary is a 20-year old Black female. She is 53
  tall and has brown eyes, black hair, and an IQ of
  142. Her mother and father are both doctors, and
  she is an only child. Mary is a history major
  with a minor in psychology. She had a head cold
  yesterday and went to bed at 8pm. She arrived at
  the experiment well rested and feeling much
  better. However, she skipped breakfast and is
  hungry.

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Why are Individual Differences Such a Problem?

• 2 Major Concerns
• Individual differences can become confounding
  variables
• Individual differences can produce high
  variability in scores

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Individual Differences as Confounding Variables

• Must ensure that different groups are as similar
  as possible, except for IV
• Categories of Confounding Variables
• Individual Differences
• Environmental Variables

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Equivalent Groups

• With a BW Subjects design, the researcher has
  control over assignment of individual to groups
• The separate groups must be

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Individual Differences and Variability An Example
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Individual Differences and Variability An
Example, cont.

• Randomly select sample of 20 individuals
  (numbers) from Population A
• Divide sample into two groups
• Assign one group to the control condition and a
  second to a treatment condition

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Individual Differences and Variability An
Example, cont.
Population A
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Individual Differences and Variability An
Example, cont.

• Randomly select sample of 20 individuals
  (numbers) from Population B
• Divide sample into two groups
• Assign one group to the control condition and a
  second to a treatment condition

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Individual Differences and Variability An
Example, cont.
Population B
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Differences between Treatments and Variability
within Treatments

• Goal of BW Ss design is to demonstrate that
  scores in one treatment condition are
  significantly different that scores in another
  condition
• Big differences conditions are good
• Big differences conditions are bad

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Minimizing Variability within Treatments

• Standardize procedures and treatment setting
• Limit individual differences
• Sample size

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Other Threats to Internal Validity

• Assignment bias
• Differential attrition
• Diffusion or imitation of treatment

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Bargh, Chen, and Burrows (1996) Study 2

• Goal Examine the influence of primes on behavior
• Ps placed in one of two conditions
• C1 rude prime
• C2 polite prime
• C3 neutral prime
• Measured how long it took Ps to interrupt
  experimenter

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Bargh, Chen, and Burrows (1996) Study 2 Results
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Thursday

• Within-subjects design
• Cozby, Chapter 8