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Evaluating importance: An overview

Evaluating importance: An overview. Size (magnitude) of effect (a.k.a. practical significance) d or other Functional significance (a.k.a. clinical significance) e.g., social validity ratings

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Evaluating importance: An overview

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  1. Evaluating importance: An overview • Size (magnitude) of effect (a.k.a. practical significance) • d or other • Functional significance (a.k.a. clinical significance) • e.g., social validity ratings • Cost-benefit ratio • Feasibility

  2. Practical vs statistical significance • Statistical significance (alpha level; p-value) reflects the odds that a particular finding could have occurred by chance. • If the p-value for a difference between two groups is 0.05, it would be expected to occur by chance just 5 times out of 100 (thus, it is likely to be a “real” difference). • If the p-value for the difference is 0.01, it would be expected to occur by chance just one time out of 100 (thus, we can be even more confident that the difference is real rather than random).

  3. Practical significance • Reflects the magnitude, or size, of the difference, not the odds that it could have occurred by chance • Arguably much more important than statistical significance, especially for clinical questions • Measures of effect size (ES) quantify practical significance of a finding

  4. Effect size • The degree to which the null hypothesis is false, e.g., not just that two groups differ significantly, but how much they differ (Cohen, 1990) • Several measures of ES exist; use “whatever conveys the magnitude of the phenomenon of interest appropriate to the research context” (Cohen, 1990, p. 1310) • IQ and height example (Cohen, 1990)

  5. The height-IQ correlation: Cohen’s (1990) example on statistical and practical significance • A study of 14,000 children ages 6-17 showed a “highly significant” (p < .001) correlation of r = .11) between height and IQ • What does this p indicate? • What’s the magnitude of this correlation? • Accounts for 1% of the variance • Based on an r this big, you’d expect that increasing a child’s height by 4 feet would increase IQ by 30 points, and that increasing IQ by 233 points would increase height by 4 inches (as a correlation, the predicted relationship could work in either direction)

  6. 2 main types of ES measures • Variance accounted for • a squared metric reflecting the percentage of variance in the dependent variable explained by the independent variable • e.g., squared correlations, odds ratios, kappa statistics • Standardized difference • scales measurements across studies into a single metric referenced to some standard deviation • d the most common and the easiest conceptually: our focus today

  7. Effect size • APA (2001) Publication Manual mandates: . . .it is almost always necessary to include some index of effect size or strength of relationship…provide the reader not only with information about statistical significance but also with enough information to assess the magnitude of the observed effect or relationship (pp. 25-26).

  8. APA guidelines (2001) mandate inclusion of ES information (not just p-value information) in all published reports • Until that happy day, if ES information is missing, readers must estimate ES for themselves • When group means and SDs are reported, you often can estimate effect size quickly and decide whether to keep reading or not

  9. Finding, estimating and interpreting d in group comparison studies • d = Difference between the means of the two groups, divided by the standard deviation (SD) • Interpret as size of group difference in SD units • When average mean difference between tx and control groups is 0.8 to 1 SD, practical significance has been defined as “high”

  10. Estimating d • Find group means, subtract them, and divide by the standard deviation. • When SDs for the groups are identical, hooray. When not, arguments have been made for using the control group SD, or the average of the two SDs. • My preference is the second, which is more conservative and strikes me as more appropriate when dealing with the large variability we see in many groups of patients with disorders

  11. Exercise 1: Calculating effect size, given group means and SDs • Data from Arnold et al. (2004) study comparing scores on SNAP composite test after four types of treatment for ADHD (Scores on SNAP composite; lower = better): Treatment group Mean (SD) Combined 0.92 (0.50) Medical management 0.95 (0.51) Behavioral 1.34 (0.56) Community care 1.40 (0.54)

  12. d demonstration, comparing SNAP performance in Combined and Medical Mgt groups Combined 0.92 (0.50) Medical management 0.95 (0.51) d = 0.92-0.95/0.505 = -.03/.505 = -.0594 Interpretation: The Combined group scored about 6/100s of a standard deviation better (lower) than the Medical Mgt group (an extremely tiny difference; these treatment approaches resulted in virtually the same outcomes on the SNAP measure)

  13. d for Combined vs Community Care treatment groups Combined 0.92 (0.50) Community care 1.40 (0.54) d = 0.92-1.40/0.52 = -0.48/.52 = -0.92 Interpretation: The Combined group scored nearly a whole standard deviation better than the Community care group; this is a large effect size. Combined treatment is substantially better than Community care.

  14. d for Medical Mgt vs Behavioral treatment Medical management 0.95 (0.51) Behavioral 1.34 (0.56) d = ? Interpretation?

  15. d for Medical Mgt vs Behavioral treatment Medical management 0.95 (0.51) Behavioral 1.34 (0.56) d = 0.95-1.34= -.39/.535 = -.72897 = -.73: Interpretation: The Medical Mgt group scored about 3/4s of a SD better than the behavioral group. This is a solid effect size suggesting that Medical Mgt treatment was substantially more effective than Behavioral treatment.

  16. Exercise 1: Interpreting d in the happy cases when it’s reported • Treatment-difference effect sizes (Cohen’s d) from Arnold et al., 2004 (Table II, p. 45) Combined vs Medical Management 0.06 Combined vs Behavioral 0.79 Combined vs Community Care 0.92 Medical Management vs Behavioral 0.72 Medical Mgt vs Community Care 0.85 Behavioral vs Community Care 0.11 • Note that our calculated ds match these.

  17. On to theme 3: an overview of evaluating precision • Precision is reflected by the width of the confidence interval (CI) surrounding a given finding • Any given finding is acknowledged to be an estimate of the “real” or “true” finding • CI reflects the range of values that includes the real finding with a known probability • A finding with a narrower CI is more precise (and thus more clinically useful) than a finding with a broader CI

  18. Evaluating precision (cont.) • CIs are calculated by adding and subtracting a multiple of the standard error for a finding/value (e.g., value + 1.96SE to determine the 95% CI) • standard error depends on sample size and reliability; larger samples and higher reliability will result in narrower CIs, all else being equal • Sackett et al. (2000) Appendix 1 shows how to calculate CIs by hand, and easy-to-use statistical programs (many free on the web) provide CIs when raw data are available.

  19. Finding and interpreting evidence of precision • CIs for difference between means of 206 children receiving early TTP and 196 receiving late TTP for OME (Paradise et al. 2001) Early Late 95% CI PPVT 92 (13) 92 (15) -2.8 to 2.8 NDW 124 (32) 126 (30) -7.6 to 4.8 PCC-R 85 (7) 86 (7) -2.1 to 0.7 • CIs are narrow thanks to large sample

  20. Contrast with risk estimates for low PCC-R from smaller samples of children with (n=15) and without (n=47) OME-associated hearing loss (Shriberg et al., 2000) • Estimated risk was 9.60 (i.e., children with hearing loss were 9.6 times more likely to have low PCC-R at age 3 than children without • But 95% confidence interval was 1.08-85.58 – meaning that this increased risk was somewhere between none and a lot. Not very precise!

  21. Predict precision • In one study, children with histories of OME (n=10) had significantly lower scores on a competitive listening task than children without OME histories (n=13) OME+ OME- p -6.8 (2.8) -9.7 (2.6) .016 • How could you quantify importance? • What would you predict about precision?

  22. When multiple studies of a question are available, meta-analysis • Quantitative summary of effects across a number of studies addressing particular question, usually in the form of a d (effect size) statistic • In EBP evidence reviews, the highest quality evidence comes from meta-analysis of studies with strong validity, precision, and importance

  23. Evidence levels for evaluating quality of treatment studiesa Best Ia Meta-analysis of >1 randomized controlled trial (RCT) Ib Well-designed randomized controlled study IIa Well-designed controlled study without randomization IIb Well-designed quasi-experimental study III Well-designed non-experimental studies, i.e., comparative, correlational, and case studies Worst IV Expert committee report, consensus conference, clinical experience of respected authorities

  24. A meta-analysis of OME and speech and language (Casby, 2001) • Casby (2001) summarized results of available studies of OME and children’s language • For global language abilities, the effect size for comparing mean language scores from children with and without OME histories was d = -.07. • Interpretation and a graphic representation

  25. A more informative graphic for meta-analyses • Shows d from each study as well as associated 95% CI.

  26. dand 95% CI boundaries for OME and vocabulary comprehension (Casby, 2001) Study Paradise 00 Black 93 Lonigan 92 Roberts 91m Upper 95% CI d Roberts 91l Lower 5% CI Teele 90 Lous 88 Teele 84 2.5 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Better with OME Worse with OME Overall d = .001

  27. The need for meta-analyses in communication disorders • Relatively few have been conducted, primarily because many studies in our literature • have not been conducted using procedures that would warrant their inclusion • may have been conducted carefully, but have not reported the information required • CONSORT (www.consort-statement.org) and STARD (Bossuyt et al., 2003) statements as one solution

  28. Given that few meta-analyses are available • Rapidly identify best available evidence addressing the foreground question • Appraise it critically with respect to validity, precision, and importance • Use CAT format to summarize your appraisal in an organized, readily accessible (and update-able) way

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