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Epidemiology Kept Simple

Epidemiology Kept Simple. Chapter 8 Measures of Association & Potential Impact. Important Jargon. Exposure (E)  an explanatory factor; any potential health determinant; the independent variable Disease (D)  the response; any health-related outcome; the dependent variable

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Epidemiology Kept Simple

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  1. Epidemiology Kept Simple Chapter 8 Measures of Association & Potential Impact

  2. Important Jargon • Exposure (E) an explanatory factor; any potential health determinant; the independent variable • Disease (D) the response; any health-related outcome; the dependent variable • Measure of association (syn. measure of effect)  a statistic that quantifies the relationship between an exposure and a disease • Measure of potential impact a statistic that quantifies the potential impact of removing a hazardous exposure Chapter 8

  3. Arithmetic (αριθμός) Comparisons • Measures of association are mathematical comparisons • Mathematic comparisons can be done in absolute terms or relative terms • Let us start with this ridiculously simple example: • I have $2 • You have $1 "For the things of this world cannot be made known without a knowledge of mathematics."- Roger Bacon Chapter 8

  4. Absolute Comparison • In absolute terms, I have $2 – $1 = $1 more than you • Note: the absolute comparison was made with subtraction It is as simple as that… Chapter 8

  5. Relative Comparison • Recall that I have $2 and you have $1. • In relative terms, I have $2 ÷ $1 = 2, or “twiceas much as you” • Note: relative comparison was made by division Chapter 8

  6. Applied to Risks • Suppose, I am exposed to a risk factor and have a 2% risk of disease. • You are not exposed and you have a 1% risk of the disease. • Of course we are assuming we are the same in every way except for this risk factor. • In absolute terms, I have 2% – 1% = 1% greater risk of the disease • This is the risk difference Chapter 8

  7. Applied to Risks • In relative terms I have 2% ÷ 1% = 2, or twice the risk • This is the relative risk associated with the exposure Chapter 8

  8. Terminology For simplicity sake, the terms “risk” and “rate” will be applied to all incidence and prevalence measures. Chapter 8

  9. Risk Difference Risk Difference (RD)  absolute effect associated with exposure where R1 ≡ risk in the exposed group R0≡risk in the non-exposed group Interpretation: Excess risk in absolute terms Chapter 8

  10. Relative Risk Relative Risk (RR)  relative effect associated with exposure or the “risk ratio” where R1 ≡ risk in the exposed group R0≡risk in the non-exposed group Interpretation: excess risk in relative terms. Chapter 8

  11. Example Fitness & Mortality (Blair et al., 1995) • Is improved fitness associated with decreased mortality? • Exposure ≡ improved fitness (1 = yes, 0 = no) • Disease ≡ death(1 = yes, 0 = no) • Mortality rate, group 1:R1 = 67.7 per 100,000 p-yrs • Mortality rate, group 0:R0 = 122.0 per 100,000 p-yrs Chapter 8

  12. ExampleRisk Difference What is the effect of improved fitness on mortality in absolute terms? The effect of the exposure (improved fitness) is to decrease mortality by 54.4 per 100,000 person-years Chapter 8

  13. ExampleRelative Risk What is the effect of improved fitness on mortality in relative terms? The effect of the exposure is to cut the risk almost in half. Chapter 8

  14. Designation of Exposure • Switching the designmation of “exposure” does not materially affect interpretations • For example, if we had let “exposure” ≡ failure to improve fitness • RR = R1 / R0 = 122.0 / 67.7 = 1.80 (1.8 times the risk in the exposed group (“almost double”) Chapter 8

  15. 2-by-2 Table Format For person-time data: let N1≡ person-time in group 1 and N0≡ person-time in group 0, and ignore cells B1 and B0 Chapter 8

  16. Fitness Data, table format Rates per 10,000 person-years Chapter 8

  17. Food borne Outbreak Example Exposure ≡ eating a particular dish Disease ≡ gastroenteritis Chapter 8

  18. Food borne Outbreak Data Exposed group had 5 times the risk Chapter 8

  19. What do you do when you have multiple levels of exposure? Compare rates to least exposed “reference” group Chapter 8

  20. The Odds Ratio Similar to a RR, but based on odds rather than risks • When the disease is rare, interpret the same way you interpret a RR • e.g. an OR of 1 means the risks are the same in the exposed and nonexposed groups “Cross-product ratio” Chapter 8

  21. Odds Ratio, ExampleMilunsky et al, 1989, Table 4NTD = Neural Tube Defect Exposed group had 0.29 times (about a quarter) the risk of the nonexposed group Chapter 8

  22. Measures of Potential Impact • These measures predicted impact of removing a hazardous exposure from the population • Two types • Attributable fraction in exposed cases • Attributable fraction in the population as a whole Chapter 8

  23. Attributable Fraction Exposed Cases (AFe) Proportion of exposed cases averted with elimination of the exposure Chapter 8

  24. Example: AFe RR of lung CA associated with moderate smoking is approx. 10.4. Therefore: Interpretation: 90.4% of lung cancer in moderate smokers would be averted if they had not smoked. Chapter 8

  25. Attributable Fraction, Population (AFp) Proportion of all cases averted with elimination of exposure from the population Chapter 8

  26. AFp equivalent formulas Chapter 8

  27. AFp for Cancer Mortality, Selected Exposures Chapter 8

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