ESTIMATION OF RISK, IS THERE AN ASSOCIATIONPSM.pptx

estimation of risk

1. Estimating Risk : Is There an Association

2. LEARNING OBJECTIVE To explore the concept of absolute risk. To introduce and compare relative risk and odds ratio as measures of association between an exposure and a disease. To calculate and interpret a relative risk in a cohort study. To calculate and interpret an odds ratio in a cohort study and in a case-control study and to describe when the odds ratio is a good estimate of the relative risk. To calculate and interpret an odds ratio in a matched-pairs case-control study.

3. BASIC STUDY DESIGNS IN EPIDEMIOLOGIC INVESTIGATIONS

4. BASIC STUDY DESIGNS IN EPIDEMIOLOGIC INVESTIGATIONS

5. ABSOLUTE RISK The incidence of a disease in a population is termed as the absolute risk. Indicates the magnitude of the risk. Does not indicate whether the exposure is associated with an increased risk of the disease. Does not stipulate any explicit comparison ; but implicit comparison.

6. HOW TO DETERMINE ASSOCIATION BETWEEN DISEASE AND EXPOSURE Eg :-Results of an investigation of a foodborne disease outbreak. Suspect foods were identified ; For each food, the attack rate of the disease for those who ate the food(exposed) The attack rate for those who didn’t eat the food(unexposed)

7. HOW TO DETERMINE ASSOCIATION BETWEEN DISEASE AND EXPOSURE To determine whether an excess risk is associated with each of the food items? One approach is---- Calculate the ratio of the attack rate in ‘exposed’ to the attack rate in ‘unexposed’, for each food. As shown in the column C of the following table-

8. HOW TO DETERMINE ASSOCIATION BETWEEN DISEASE AND EXPOSURE

9. HOW TO DETERMINE ASSOCIATION BETWEEN DISEASE AND EXPOSURE Alternate approach--- As shown in column D Subtract the risk in the ‘unexposed’ from the risk in ‘exposed’. The difference represents the excess absolute risk in those who were exposed. Thus an excess risk can be calculated as : The ratio of risks(or of incidence rates) : Disease risk in exposed/ Disease risk in unexposed The difference in the risks(or in the incidence rates) : (Disease risk in exposed) – (Disease risk in unexposed)

10. DIFFERENCE IN BOTH THE WAYS OF CALCULATION Does the method that we choose to calculate excess risk make any difference? Another example-Two communities, A and B. Incidence if a disease in exposed in A is 40% ; In unexposed-10% Ratio of the rates= 4.0 Difference in incidence rates=30%. Similarly in B- Ratio -> 90/60=1.5 Difference in incidence=30%

11. RELATIVE RISK Both case control and cohort studies designed to determine if there is an association between exposure and disease. As per Cohort study, question would be- Relative risk = Risk in exposed/Risk in unexposed It can also be defined as Probability of an event occurring in exposed people compared with the probability of the event in unexposed people ; as the ratio of these two probabilities .

12. INTERPRETING THE RELATIVE RISK If the Relative risk = 1 No evidence for any increased risk in exposed individuals . If Relative risk >1 Evidence of positive association , which may be causal. If Relative risk <1 Evidence of negative association ; protective effect. People who are given an effective vaccine

13. CALCULATION OF RELATIVE RISK IN COHORT STUDIES

14. CALCULATION OF RELATIVE RISK IN COHORT STUDIES

15. RELATIVE RISK : EXAMPLE

16. RELATIVE RISK : EXAMPLE This Fig shows the data based on merging 2,282 middle aged men followed for 10yrs in the Framingham study 1,838 middle aged men followed for 8years in Albany, New York. Data relate smoking, cholesterol level, BP to risk of Myocardial Infarction and Death from CHD. The value of 1 is assigned to the LOWEST of the risk ; other risks are calculated relative to this figure. Left pic- shows risk in non smoker with low cholesterol level(set to be as 1) Both smoking and elevated cholesterol levels contribute to the risk of MI and death from CHD. Right pic- Comparable analysis of BP and smoking

17. ODDS RATIO(RELATIVE ODDS) In a Case-Control study , Incidence of disease in exposed or in unexposed is not known because We start with diseased people(cases) and non diseased people(control). Relative risk cannot be calculated directly; but Odds Ratio. OR can be obtained from either of the study(cohort/case control study)

18. ODDS RATIO(RELATIVE ODDS) Proportion of the cases exposed and the proportion of controls who were exposed.

19. ODDS RATIO(RELATIVE ODDS) Odds = Probability that Epi Beauty will win the race/ Probability that Epi Beauty will lose the race P=Probability that Epi will win the race. 1-p= Probability that Epi Beauty will lose the race. Odds =p/1-p Probability of winning=60% Odds of winning=60%/40%= 1.5

20. ODDS RATIO IN COHORT AND CASE CONTROL STUDIES

21. INTERPRETING ODDS RATIO When is the ODDS ratio a good estimate of the Relative Risk

22. CALCULATION OF ODDS RATIO IN AN UNMATCHED CASE CONTROL STUDY

23. CALCULATION IN MATCHED PAIRS CASE CONTROL STUDY