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Effect Size – Ch. 4

Effect Size – Ch. 4. Effect Size is to Difference Between Means as Z i is to X i An easier way to assess the magnitude of X-bar 1 -Xbar 2 Looks at the difference between means relative to differences between individuals within each group

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Effect Size – Ch. 4

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  1. Effect Size – Ch. 4 • Effect Size is to Difference Between Means as Zi is to Xi • An easier way to assess the magnitude of X-bar1-Xbar2 • Looks at the difference between means relative to • differences between individuals within each group • Are groups differences large relative to individual differences? • E.G.: Cognitive ability is 13 points higher in first-born relative • to second-born children • Is this an important difference?

  2. Effect Size, cont. • Numerator: Difference Between Groups (Means) • Denominator: Differences AmongSubjects Within each Group • Ratio: Is a Comparison of Differences Between to Differences Within • (Absolute Value) • Think Ahead: t-tests and ANOVAs compare differences Between • To differences Within

  3. Large Effect --13 pt. Difference-- d is large

  4. Small Effect 13 pt. Difference   d is small

  5. The Two Ts of Research:True but Trivial T T

  6. Interpreting d Small Effect: d=0.20 Medium Effect: d=0.50 Large Effect: d=0.80

  7. What Does p<0.05 Mean? “Significant”: Signifies a relationship, does not imply that that relationship is large or important.

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