Analysis and Interpretation
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Cochrane Review author training workshop, January 22-23, 2009 at the University of Calgary Health Sciences Centre
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Analysis and Interpretation
- 7. Risk ratio (RR) aka relative risk RR = a / (a+b) c / (c+d) Risk/ probability/ chance of the occurrence of an event in treatment relative to control Intervention Control a+b=n I c+d=n C Event No event d c b a
- 8. Sample RR Calculation Death No death RR = 14/133 = 0.11 = 0.13 128/148 0.86 Drug 133 148 Placebo 20 128 119 14
- 9. Odds ratio (OR) Intervention Control No event Event OR = a / b c / d Odds of an event occurring to it not occurring for treatment relative to control a+b=n I c+d=n C d c b a
- 10. Sample OR Calculation Death No death Drug Placebo 133 148 OR = 14/119 = 0.12 = 0.019 128/20 6.4 20 128 119 14
- 11. Interpreting (for intervention) Increased odds (harmful) Increased odds (beneficial) OR>1 (6.4/0.12) Reduced odds (beneficial) Reduced odds (not beneficial) OR<1 (0.12/6.4) No difference No difference OR=1, RR=1 Increased risk (harmful) Increased risk (beneficial) RR>1 (0.86/0.11) Reduced risk (beneficial) Reduced risk (not beneficial) RR<1 (0.11/0.86) Bad outcome (e.g. infection) Good outcome (e.g. remission)
- 13. Closer Look at Odds RR = 0.11 / 0.86 = 0.13 ↑ A rate (11%) OR = 0.12 / 6.4 = 0.019 ↑ ~1:9 ↑ ~7:1
- 15. Risk Difference (RD) Death No death Actual difference in risk of events Placebo Drug 133 148 RD = 14/133 – 128/148 = 0.11 – 0.86 = - 0.75 20 128 119 14
- 23. Fixed Effects Analysis in Picture View
- 24. Random Effects Analysis in Picture View
- 25. Random effects in RevMan 5 ← DerSimonian and Laird random effects model
- 26. Random effects in RevMan 5 (continued) ← DerSimonian and Laird random effects model
- 36. I 2 Statistic (continued) * Importance of I 2 value depends on: ● magnitude and direction of effects ● strength of evidence of heterogeneity - Chi-squared P value, or - I 2 confidence interval Considerable heterogeneity* 75% to 100% May represent substantial heterogeneity* 50% to 90% May represent moderate heterogeneity* 30% to 60% Might not be important 0% to 40% Guide to Interpretation I 2 value
- 37. Sample Forest Plot: Q and I 2