BHS 204-01 Methods in Behavioral Sciences I

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BHS 204-01Methods in Behavioral Sciences I

• May 9, 2003
• Chapter 6 and 7 (Ray)
• Control The Keystone of the Experimental Method

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Sources of Variance

• Systematic variation differences related to the
  experimental manipulation.
• Can also be differences related to uncontrolled
  variables (confounds) or systematic bias (e.g.
  faulty equipment or procedures).
• Chance variation nonsystematic differences.
• Cannot be attributed to any factor.
• Also called error.

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F-Ratio

• A comparison of the differences between groups
  with the differences within groups.
• Between-group variance treatment effect
  chance variance.
• Within-group variance chance variance.
• If there is a treatment effect, then the
  between-group variance should be greater than the
  within-group variance.

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Testing the Null Hypothesis

• Between-group variance (treatment effect) must be
  greater than within-group variance (chance
  variation), F gt 1.0.
• How much greater?
• Normal curve shows that 2 SD, p lt.05 is likely to
  be a meaningful difference.
• The p value is a compromise between the
  likelihood of accepting a false finding and the
  likelihood of not accepting a true hypothesis.

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Box 6.1. (p. 135)Type I and Type II Errors.
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Types of Errors

• Type I error likelihood of rejecting the null
  when it is true and accepting the alternative
  when it is false (making a false claim).
• This is the p value -- .05 is probability of
  making a Type I error.
• Type II error likelihood of accepting null when
  it is false and rejecting the alternative when it
  is true.
• Probability is b, the power of a statistic is 1-b.

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Reporting the F-Ratio

• ANOVA is used to calculate the F-Ratio.
• Example
• The experimental group showed significantly
  greater weight gain (M 55) compared to the
  control group (M 21), F(1, 12) 4.75, p.05.
• Give the degrees of freedom for the numerator and
  denominator.

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When to Use ANOVA

• When there are two or more independent groups.
• When the population is likely to be normally
  distributed.
• When variance is similar within the groups
  compared.
• When group sizes (Ns) are close to equal.

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Threats to Internal Validity

• It is the experimenters job to eliminate as many
  threats to internal validity as possible.
• Such threats constitute sources of systematic
  variance that can be confused with an effect,
  resulting in a Type I error.
• Potential threats to validity must be evaluated
  in the Discussion section of the research report.

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Two Ways of Achieving Control

• Participant assignment and selection
• Random sampling.
• Random assignment to conditions.
• Experimental design
• Add a control group.
• Include a baseline measurement before the
  treatment (pretest).
• Treat subjects consistently across all groups.
• Four-group design tests for effects of the
  testing.

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Figure 7.3. (p. 159)The four conceptual steps
in experimentation.