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 Type II 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 Significance level (alpha)
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