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POST HOC COMPARISONS

POST HOC COMPARISONS. What is the Purpose?. A significant F in ANOVA tells you only that there is a difference among the groups, not which groups are different. Post hoc tests can be done to make specific comparisons. Post hocs also control experimentwise alpha level.

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POST HOC COMPARISONS

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  1. POST HOC COMPARISONS

  2. What is the Purpose? • A significant F in ANOVA tells you only that there is a difference among the groups, not which groups are different. • Post hoc tests can be done to make specific comparisons. • Post hocs also control experimentwise alpha level.

  3. How Do Post Hoc Methods Differ? • Some post hocs are more conservative, meaning careful about avoiding Type I error. • Post hocs methods also differ in what type of comparisons they allow • Pairwise: one condition compared to another • Nonpairwise: combinations of conditions

  4. How Do Post Hoc Methods Differ? • Commonly used post hoc methods: • Fisher’s PLSD • Scheffe • Tukey’s HSD

  5. Fisher’s PLSD • Protected Least Significant Difference • Less conservative • F-test in ANOVA must be significant • Pairwise comparisons only

  6. Scheffe • Very conservative • Pairwise or nonpairwise comparisons • High critical value in exchange for allowing any comparison

  7. Tukey’s HSD • Medium conservative • Pairwise comparisons only • Each group must have same n

  8. How Does the Tukey’s HSD Work? • Honestly Significant Difference is calculated. • Any difference between means must be larger than the HSD to be significant. • Larger differences are needed as more groups are being compared; this controls Type I error rate.

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