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Measures of risk and association

Measures of risk and association. Risk. Likelihood of having a disease or any health phenomenon. - How can you measure the risk? Risk is measured by Rate (incidence, prevalence, mortality,…..) - How do you know that this risk is high or low? By risk comparison.

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Measures of risk and association

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  1. Measures of risk and association

  2. Risk Likelihood of having a disease or any health phenomenon. - How can you measure the risk? Risk is measured by Rate (incidence, prevalence, mortality,…..) - How do you know that this risk is high or low? By risk comparison

  3. Examples of risk comparison: risk ratio (relative risk), attributable risk, rate ratio, odds ratio.

  4. Relative Risk (Risk Ratio) • It compares the risk of some health-related event such as disease or death in two groups regarding risk factor. • RISK FACTOR: sex, age, race, or exposure to any suspected risk (SE, food, chemical, behavior ….. ). • The group of primary interest is labeled as “exposed” group, and the comparison group is labeled as “unexposed” group. • “exposed” group .. numerator; • “unexposed” group ..denominator: Risk Ratio (RR) = incidence among exposed group incidence among non exposed

  5. Relative Risk (Risk Ratio) • Relative risk ( RR),Risk Ratio • Estimates the magnitude of an association between exposure & disease. • Which is important in the causation & in the policy of prevention.

  6. Start Outcome Family smoker 500 children Exposed Diseased 300 Children (<12 yrs) 1000 Not diseased 200 1 year Diseased 120 Family non-smoker 500 children Not exposed Not diseased 380

  7. Rate: Incidence rate • Incidence of Resp. Infection among exposed children: 300 • 500 * 100 = 60% • Incidence of Resp. Infect. Among non exposed children: 120 • 500 *100 = 24%

  8. Cohort Study(cont.) Relative RiskIncidence rate among exposed Risk Ratio = Incidence rate in non exposed. 60% 24% = 2.5 ? RR = 1: means zero (no effect of the exposure) RR > 1: means (positive effect of the exposure) RR < 1: means negative effect of exposure (protective)

  9. Attributable risk (AR), Risk difference: • Excess risk of the disease in those exposed compared with those none exposed. • AR = Ie – Io

  10. Base-line risk Incidence A R I. among non exposed I. among exposed

  11. Attributable Risk: 60% - 24% = 36% 36% of Resp. Infection among <12 children can be attributed to family smoking.

  12. AR%(Attributable proportion): • pop. risk • The percentage of cases which could be prevented if we remove the exposure to that factor. • AR% = Ie – Io * 100 Ie AR% = 36% / 60% = 60% ?

  13. Rate Ratio A rate ratio compares two groups in terms of , person-time rates, or mortality rates. Rate ratio = rate for group of primary interest rate for comparison group The interpretation of the value of a rate ratio is similar to that of the risk ratio.

  14. Doll and Hill study : Mortality of British doctors cited from Mausner, 1985

  15. Odds ratio • Is another measure of association. • It is a measure of choice for case-control study. • quantifies the relationship between an exposure and health outcome from a comparative study. The odds ratio is calculated as: Odds ratio = ad bc a = No. of persons with disease and with exposure of interest b = No. of persons without disease, but with exposure of interest c = No. of persons with disease, but without exposure of interest d = No. of persons without disease & without exposure of interest a + c = total number of persons with disease (“cases”) b + d = total number of persons without disease (“controls”)

  16. Case Control Studies CROSS -RATIO

  17. The measure of association between the risk factor and the disease in a case-control is calculated by “odds ratio” (OR)“Odds of factor among cases divided by odds of factor among control”OR =

  18. Example: A study of TB with certain exposure in a Hypothetical case-control study. RiskFactor OR= 100x140 / 40x80 = 4.4 TB was 4 times more among those with bad nutrition than those who are not.

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