1 / 17

IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #27

IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #27. Comparison of Observational Studies. Statistical Analyses - 1 (Cross-Sectional & Case-Control). Prevalence = proportion of cases p a = a/c 1 = exposed to risk factor p b = b/c 2 = not exposed to risk factor.

paula-harris
Download Presentation

IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #27

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. IE 552 Mechanics of Musculoskeletal System Dr. Andris Freivalds Class #27 IE 552

  2. Comparison of Observational Studies IE 552

  3. Statistical Analyses - 1 (Cross-Sectional & Case-Control) • Prevalence = proportion of cases • pa = a/c1 = exposed to risk factor • pb = b/c2 = not exposed to risk factor IE 552

  4. Statistical Analyses - 2 • Prevalence difference = pa -pb • Confidence interval = pa –pb + zα/2[pa(1- pa)/c1 + [pb(1- pb)/c2]½ IE 552

  5. Normal Distribution IE 552

  6. Statistical Analyses - Odds Ratio • OR = ratio of prevalence odds for exposed ratio of prevalence odds for unexposed • OR = pa/(1- pa) = ad/bc pb/(1- pb) IE 552

  7. Contingency Table in a Case-Control Study IE 552

  8. IE 552

  9. Statistical Analyses - Odds Ratio • Confidence interval = OR e(+zα/2 SE) • SE = (1/a + 1/b + 1/c + 1/d)½ IE 552

  10. Statistical Analyses – Chi Square • χ2 = ∑(oi-ei)2/ei • o = observed values • e = expected values • ea = c1/n x r1/n x n = c1r1/n IE 552

  11. Statistical Analyses – Chi Square • Compare to χ2α/2 with • degrees of freedom = (r-1)(c-1) • χ2 = n(ad – bc)2 c1c2r1r2 IE 552

  12. Chi-Square Critical Values IE 552

  13. Statistical Analyses (Cohort Studies) • Relative Risk or Risk Ratio (RR) • Relates risk for exposed compared to unexposed groups • Rexp = # exposed incurring injury = a/(a+c) all exposed individuals • Runexp = b/(b+d) • RR = Rexp/Runexp = a(b+d) b(a+c) IE 552

  14. Contingency Table in a Cohort Study IE 552

  15. IE 552

  16. Statistical Analyses - 2 • RR confidence interval = RR e(+zα/2 SE) • SE = │ c + d │½ │ a(a+c) b(b+d)│ • Attributable Risk (AR) = (RR -1)/RR IE 552

  17. RR vs. OR • Case-control studies • RR?? but disease didn’t cause risk • OR better, snapshot in time • Cohort studies • Both measure the association between exposure and risk • OR = ad/bc ≈ a(b+d)/b(a+c) = RR • If a & b are small wrt c & d IE 552

More Related