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Statistics for clinicians

Delve into attribution methods for public health outcomes and learn to calculate impact metrics using real-world examples. This section explores the significance of Relative Risk and Attributable Risk in epidemiological research.

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Statistics for clinicians

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  1. Statistics for clinicians Biostatistics course by Kevin E. Kip, Ph.D., FAHAProfessor and Executive Director, Research CenterUniversity of South Florida, College of NursingProfessor, College of Public HealthDepartment of Epidemiology and BiostatisticsAssociate Member, Byrd Alzheimer’s InstituteMorsani College of MedicineTampa, FL, USA

  2. SECTION 7.1 Introduction to Meta-Analysis Meta analysis and measures of public health impact

  3. SECTION 7.6 Introduction to Measures of Public Health Impact

  4. Learning Outcome: Explain features of individual measures of public health impact.

  5. Measures of Public Health Impact Attributable Risk (AR) Number Attributable Risk Percent (AR%) Percentage Population Attributable Risk (PAR) Number Population Attributable Risk Percent (PAR%) Percentage Number Needed to Treat (NNT) Number Number Needed to Harm (NNH) Number

  6. Measures of Public Health Impact IMPORTANT! They all assume (require) that a cause-effect relationship exists between the exposure and the outcome.

  7. Relative Risk vs. Attributable Risk Relative Risk: Measure of the strength of association, and indicator used to assess the possibility of a causal relationship. Attributable Risk: Measure of the potential for prevention of disease if the exposure could be eliminated (assuming a causal relationship).

  8. Relative Risk vs. Attributable Risk Relative Risk: • Etiology Attributable Risk: • Policy decisions • Funding decisions (e.g. prevention programs)

  9. Measures of Public Health Impact Attributable Risk: Refers to EXPOSED persons. Population Attributable Risk: Refers to both EXPOSED and NONEXPOSED persons.

  10. SECTION 7.7 Attributable Risk And Attributable Risk Percent

  11. Learning Outcome: Calculate and interpret the measures attributable risk (AR) and attributable risk percent (AR%)

  12. Attributable Risk (AR) Among the EXPOSED: How much of the disease that occurs can be attributed to a certain exposure? AR AR% This is of primary interest to the practicing clinician.

  13. Attributable Risk (AR) AR = Iexposed – Inonexposed = “Risk Difference” ISM = 84 / 3000 = 0.028 = 28.0 / 1000 INS = 87 / 5000 = 0.0174 = 17.4 / 1000 (background risk) Develop CHD AR = (28.0 – 17.4) / 1000 = 10.6 / 1000

  14. Attributable Risk (AR) AR = (28.0 – 17.4) / 1000 = 10.6 / 1000 Among SMOKERS, 10.6 of the 28/1000 incident cases of CHD are attributed to the fact that these people smoke … Among SMOKERS, 10.6 of the 28/1000 incident cases of CHD that occur could be prevented if smoking were eliminated.

  15. Practice - Attributable Risk (AR) Among smokers, calculate and interpret incident cases of hypertension that are attributed to their smoking. AR = Iexposed – Inonexposed Hypertension Among SMOKERS, Among SMOKERS,

  16. Practice - Attributable Risk (AR) Among smokers, calculate and interpret incident cases of hypertension that are attributed to their smoking. AR = Iexposed – Inonexposed AR = 0.130 – 0.096 = 0.034 Hypertension Among SMOKERS, 34.2 of the 130/1000 incident cases of hypertension are attributed to the fact that these people smoke … Among SMOKERS, 34.2 of the 130/1000 incident cases of hypertension that occur could be preventedif smoking were eliminated.

  17. Attributable Risk Percent (AR%) AR% = (Iexposed – Inonexposed) / Iexposed = “Etiologic fraction” ISM = 84 / 3000 = 0.028 = 28.0 / 1000 INS = 87 / 5000 = 0.0174 = 17.4 / 1000 (background risk) Develop CHD AR% = (28.0 – 17.4) / 28.0 = 37.9%

  18. Attributable Risk Percent (AR%) AR% = (28.0 – 17.4) / 28.0 = 37.9% Among SMOKERS, 38% of the morbidity from CHD may be attributed to smoking… Among SMOKERS, 38% of the morbidity from CHD could be prevented if smoking were eliminated.

  19. Practice - Attributable Risk Percent (AR%) Among heavy drinkers, calculate and interpret the percentage of incident cases of the metabolic syndrome attributed to their drinking. AR% = (Iexposed – Inonexposed ) / Iexposed AR% = Metabolic Syndrome Among HEAVY DRINKERS, Among HEAVY DRINKERS,

  20. Practice - Attributable Risk Percent (AR%) Among heavy drinkers, calculate and interpret the percentage of incident cases of the metabolic syndrome attributed to their drinking. AR% = (Iexposed – Inonexposed ) / Iexposed AR% = (0.119 – 0.067) / 0.119 = 0.439 x 100 = 43.9% Metabolic Syndrome Among HEAVY DRINKERS, 43.9% of the morbidity from the metabolic syndrome may be attributed to heaving drinking … Among HEAVY DRINKERS, 43.9% of the morbidity from the metabolic syndrome could be preventedif heavy drinking were eliminated.

  21. SECTION 7.8 Population Attributable Risk and Population Attributable Risk Percent

  22. Learning Outcome: Calculate and interpret the measures population attributable risk (PAR) and population attributable risk percent (PAR%)

  23. Population Attributable Risk (PAR) Among the EXPOSED and NONEXPOSED (e.g. total population): How much of the disease that occurs can be attributed to a certain exposure? PAR PAR% This of interest to policy makers and those responsible for funding prevention programs.

  24. PAR and PAR% Example: We want to estimate how much of the burden of diabetes among Tampanians is attributed to obesity.

  25. PAR and PAR% CAUTION! In order to calculate PAR and PAR%, we have to be reasonably sure that the results of the study can be generalized to the population of Tampa. (e.g the incidence rates drawn from the sample approximate the incidence rates in the entire population).

  26. Population Attributable Risk (PAR) PAR = Itotal – Inonexposed IT = 1100 / 10000 = 0.11 = 110 / 1000 INE = 250 / 5500 = 0.0455 = 45.5 / 1000 (background risk) Diabetes PAR = (110 – 45.5) / 1000 = 64.5 / 1000

  27. Population Attributable Risk (PAR) PAR = (110 – 45.5) / 1000 = 64.5 / 1000 In Tampa, 64.5 of the 110/1000 incident cases of diabetes are attributed to obesity … In Tampa, 64.5 of the 110/1000 incident cases of diabetes that occur could be prevented with sufficient weight loss.

  28. Practice – Population Attributable Risk (PAR) Estimate how much of the burden of hypertension among Tampanians is attributed to high salt consumption. PAR = (Itotal – Inonexposed) PAR = Hypertension In TAMPA, In TAMPA,

  29. Practice – Population Attributable Risk (PAR) Estimate how much of the burden of hypertension among Tampanians is attributed to high salt consumption. PAR = (Itotal – Inonexposed) PAR = (0.064 – 0.051) / 0.01346 = 13.46 per 1,000 Hypertension In TAMPA, 13.46 of the 64.18/1,000 incident cases of hypertension are attributed to having high salt consumption. In TAMPA, 13.46 of the 64.18/1,000 incident cases of hypertension could be preventedwith elimination of high salt intake.

  30. Population Attributable Risk Percent PAR% = (Itotal – Inonexposed) / Itotal IT = 1100 / 10000 = 0.11 = 110 / 1000 INE = 250 / 5500 = 0.0455 = 45.5 / 1000 (background risk) Diabetes PAR% = (110 – 45.5) / 110 = 58.6%

  31. Population Attributable Risk Percent PAR% = (110 – 45.5) / 110 = 58.6% In Tampa, 59% of the cases of diabetes may be attributed to obesity in the population… In Tampa, 59% of the cases of diabetes could be prevented if Tampa residents lost sufficient weight.

  32. Practice – Population Attributable Risk% (PAR%) Estimate the percentage of the burden of hypertension among Tampanians that is attributed to high salt consumption. PAR% = (Itotal – Inonexposed) / Itotal PAR% = Hypertension In TAMPA, 21% of the cases of hypertension may be attributed to having high salt consumption in the population. In TAMPA, 21% of the cases of hypertension could be preventedwith elimination of high salt intake.

  33. Practice – Population Attributable Risk% (PAR%) Estimate the percentage of the burden of hypertension among Tampanians that is attributed to high salt consumption. PAR% = (Itotal – Inonexposed) / Itotal PAR% = (0.064 – 0.051) / 0.064 = 0.013 / 0.064 = 21.0% Hypertension In TAMPA, 21% of the cases of hypertension may be attributed to having high salt consumption in the population. In TAMPA, 21% of the cases of hypertension could be preventedwith elimination of high salt intake.

  34. Measures of Public Health Impact NOTE! Both attributable and population attributable risks should be cautiously interpreted. In reality, even if an exposure is causal, we do not know whether it truly contributed to disease occurrence in all exposed persons -- in some exposed persons, other causal factors may have been entirely responsible.

  35. SECTION 7.9 Attributable Risk and Population Attributable Risk Percent for Case-Control Studies

  36. Learning Outcome: Calculate and interpret the measures attributable risk (AR) and attributable risk percent (AR%) in Case Control Studies

  37. Measures of Public Health Impact • They are based on measures of incidence. • We can calculate incidence measures from case-control studies only under special circumstances. • Therefore, the AR and PAR cannot usually be calculated from case-control data. • However, for most case-control studies, we can calculate the AR% and PAR%.

  38. AR% (Case-Control Studies) (OR – 1) AR% = ----------- x 100 OR

  39. Example: AR% (Case-Control Studies) Case-control study to evaluate the impact of smoking as related to bladder cancer. Bladder Cancer (160 / 90) OR = ------------ (120 / 200) = 2.96

  40. Example: AR% (Case-Control Studies) Question:Among smokers, what proportion (percent) of bladder cancer cases can be attributed to their smoking habit? (OR – 1) AR% = ----------- x 100 OR AR% = ((2.96 – 1) / 2.96) x 100 = 66.2%

  41. Example: AR% (Case-Control Studies) • 66% of bladder cancer cases among smokers can be attributed to their smoking. • 66% of bladder cancer cases among smokers could be prevented if they had never taken up smoking. (Assuming there is a causal association between smoking and the development of bladder cancer).

  42. Practice: AR% (Case-Control Studies) Among smokers, estimate the percentage of esophageal cancer cases attributed to smoking. OR = _____________ (OR – 1) AR% = ----------- x 100 AR = ----------- x 100 = _______ OR Esophageal Cancer Among smokers, Among smokers,

  43. Practice: AR% (Case-Control Studies) Among smokers, estimate the percentage of esophageal cancer cases attributed to smoking. OR = (65/288) / (52/396) = 1.692 (OR – 1) (1.692 – 1) AR% = ----------- x 100 AR = ----------- x 100 = 40.9% OR 1.692 Esophageal Cancer Among smokers, 66% of esophageal cancer cases can be attributed to their smoking. Among smokers, 66% of esophageal cancer cases could be preventedif they had never taken up smoking.

  44. PAR% (Case-Control Studies) (PE) (OR – 1) PAR% = -------------------- x 100 [(PE) (OR-1)] + 1 where PE = proportion of exposed controls (assuming that the proportion of exposed controls is representative of the proportion exposed in the source population)

  45. Example: PAR% (Case-Control Studies) Case-control study to evaluate the impact of smoking as related to bladder cancer. (160 / 90) OR = ------------ (120 / 200) = 2.96 PE = 120 / 320 = 0.375 Bladder Cancer

  46. Example: PAR% (Case-Control Studies) Question:In this study population, what proportion (percent) of bladder cancer cases can be attributed to smoking? (PE) (OR – 1) PAR% = ---------------------- x 100 [(PE) (OR-1)] + 1 PAR% = (0.375) (2.96-1) [(0.375) (2.96-1)] + 1 x 100 = 42.4%

  47. Example: PAR% (Case-Control Studies) • In this study population, 42% of bladder cancer cases can be attributed to smoking. • In this study population, 42% of bladder cancer cases could be prevented if people would not take up smoking. (Assuming there is a causal association between smoking and the development of bladder cancer).

  48. Practice: PAR% (Case-Control Studies) Among the study population, estimate the percentage of esophageal cancer cases attributed to smoking. (PE) (OR – 1) PAR% = -------------------- x 100 [(PE) (OR-1)] + 1 OR = _____________ PE = _____________ PAR% = ----------- x 100 PAR% = ______________ Esophageal Cancer In this study population, In this study population,

  49. Practice: PAR% (Case-Control Studies) Among the study population, estimate the percentage of esophageal cancer cases attributed to smoking. (PE) (OR – 1) PAR% = -------------------- x 100 [(PE) (OR-1)] + 1 OR = (65 / 266) / (52 / 396) = 1.692 PE = 52 / (52 + 396) = 0.116 0.116 x (1.692 – 1) PAR% = ---------------------- x 100 PAR% = 7.4% [(0.116) (1.692-1)] + 1 Esophageal Cancer In this study population, 7.4% of esophageal cancer cases can be attributed to smoking. In this study population, 7.4% of esophageal cancer cases could be prevented if people would not take up smoking.

  50. SECTION 7.10 Attributable Risk versus Relative Risk

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