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Chapter 8

Chapter 8. Making Sense of Statistical Significance: Effect Size, Decision Errors, and Statistical Power. Effect Size. Amount that two populations do not overlap Figuring effect size ( d ) Effect size conventions small d = .2 medium d = .5 large d = .8. Meta-Analysis.

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Chapter 8

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  1. Chapter 8 Making Sense of Statistical Significance: Effect Size, Decision Errors, and Statistical Power

  2. Effect Size • Amount that two populations do not overlap • Figuring effect size (d) • Effect size conventions • small d = .2 • medium d = .5 • large d = .8

  3. Meta-Analysis • Combines results from different studies • Provides an overall effect size • Common in the more applied areas of psychology

  4. Decision Errors • Type I error • Reject the null hypothesis when in fact it is true • alpha (α) • Probability of making a Type I error • Type II error • Not rejecting the null hypothesis when in reality it is false • beta (β) • Probability of making a Type II error

  5. Possible Correct and Incorrect Decisions in Hypothesis Testing

  6. Statistical Power • Probability that the study will produce a statistically significant results if the research hypothesis is true

  7. Statistical Power • Steps for figuring power 1. Gather the needed information: mean and standard deviation of Population 2 and the predicted mean of Population 1 2. Figure the raw-score cutoff point on the comparison distribution to reject the null hypothesis

  8. Statistical Power • Steps for figuring power 3. Figure the Z score for this same point, but on the distribution of means for Population 1 4. Use the normal curve table to figure the probability of getting a score more extreme than that Z score

  9. Influences on Power • Effect size • Difference between the population means • Population standard deviation • Figuring power from predicted effect sizes

  10. Influences on Power • Sample size • Affects the standard deviation of the distribution of means • Significance level (alpha) • One- versus two-tailed tests • Type of hypothesis-testing procedure

  11. Summary of Influences on Power

  12. Practical Ways of Increasing the Power of a Planned Study

  13. Importance of Power When Evaluating Study Results • When a result is significant • Statistical significance versus practical significance • When a result if not statistically significant

  14. Controversies and Limitations • Effect size versus statistical significance • Theoretically oriented psychologists emphasize significance • Applied researchers emphasize effect size

  15. Reporting in Research Articles • Increasingly common for effect sizes to be reported • Commonly reported in meta-analyses

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