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. risk ratio (relative risk), attributable risk, rate ratio, odds ratio.
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 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

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

8. Cohort Study (cont.) Relative Risk Incidence rate among exposed
Risk Ratio =
Incidence rate in non exposed.
60%
24% = ?
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.)

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
exposed
I. among non exposed

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

12. 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% ?

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. 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

15. 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”)

16. 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

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.
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.