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

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

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

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.

Start Outcome 500 children 300 Children (<12 yrs) 1000 200 120 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

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%

Cohort Study (cont.) Relative Risk Incidence 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) Cohort Study (cont.)

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

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

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

AR% = Ie – Io * 100 Ie AR% = 36% / 60% = 60% ? 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% ?

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.

Lung Cancer 166 7 166 / 7 = 23.7 Annual Death Rates / 100,000 persons Exposure Category Lung Cancer 166 7 166 / 7 = 23.7 Heavy smokers Nonsmokers Measures of Risks Rate ratio: Doll and Hill study : Mortality of British doctors cited from Mausner, 1985

Is another measure of association. 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”)

Case Control Studies a b c d a+c b+d Leukemia Patients Controls Benzene Exposed a b Benzene non Exposed c d a+c b+d CROSS -RATIO

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 =

Example: A study of TB with certain exposure in a Hypothetical case-control study. HEALTHY TB b=40 a=100 Present d=140 c=80 Absent b + d 180 a + c Risk Factor OR= 100x140 / 40x80 = 4.4 TB was 4 times more among those with bad nutrition than those who are not.