Other Risk Estimates Attributable Risk Population Attributable Risk

Other Risk Estimates: Attributable Risk & Population Attributable Risk Sharon L. R. Kardia

How do we estimate public health effect of an exposure? Answer: Attributable Risk Statistics Attributable Risk Estimates of Effect o Attributable Risk (AR) o Attributable Risk Percent (AR%) o Population Attributable Risk (PAR) o Population Attributable Risk Percent (PAR%) These statistics address the question: o How much of the disease that occurs can be attributed to a certain exposure? 2

Why would attributing disease to a specific exposure be useful for Public Health? o If the exposure causes increased risk of disease, then we can estimate how many cases of disease could be eliminated if we completely eliminate the exposure. 3

Measures of Attribution and Effect • Attributable Risk (AR) is a Risk Difference (RD) n n It estimates the excess risk of disease in those exposed compared with those non-exposed. AR = Incidence Exposed – Incidence Not Exposed o Group of Interest: Exposed o Quantifies the risk of disease in the “exposed” group attributable to the exposure 4

AR removes “background” risk Figure 12 -1 A, Total risks in exposed and nonexposed groups. B, Background risk. C, Incidence attributable to exposure and incidence not attributable to exposure. 5

Calculating Attributable Risk o o Subtract away the background risk (i. e. incidence in the unexposed) Two measures of incidence: Incidence Rate (IR) or Cumulative Incidence (CI) For Incidence Rate: AR = IRExposed – IR Unexposed For Cumulative Incidence: AR = CIE – CIU 6

Assumptions o Interpretation of the AR is dependent on the assumption that a cause-effect relationship exists between exposure and disease. o If no association between the exposure and disease, – IU= 0 and therefore AR = 0. IE 7

How does AR compare to other measures of association? o The RR is a measure of the strength of the association and the possibility of a causal relationship. o The AR indicates the potential for prevention, if the exposure could be eliminated. 8

Example: Smoking and CHD Exposure CHD yes CHD no Total Smokes 84 2, 916 3, 000 Doesn’t smoke 87 4, 913 5, 000 Total 171 7, 829 8, 000 CIE = 84/3000 = 28. 0 per 1000 CIU = 87/5000= 17. 4 per 1000 AR = CIE-CIU = 28. 0 - 17. 4 = 10. 6 per 1000 Interpretation: The excess occurrence of CHD among smokers attributable to their smoking is 10. 6 per 1, 000. 9

Attributable Risk Percent (AR%) o What proportion of cases in exposed persons is due to the exposure? o AR% = ( IE – IU )/ (IE) o Can be interpreted as the proportion of the disease in the exposed that could be prevented by eliminating the exposure 10

How to Calculate AR%? AR% = (IRE – IRU)/ IRE or AR% = (CIE – CIU)/CIE or AR% = [(RR-1)/RR]*100 11

Recall Smoking and CHD o AR = CIE-CIU = 28. 0 - 17. 4= 10. 6 per 1000 AR% = (CIE – CIU)/CIE = (28. 0 - 17. 4)/28. 0 = 10. 6/28. 0 = 37. 9% Interpretation: If smoking does cause CHD in this population, 37. 9% of CHD among smokers can be attributed to their smoking. OR In this population, if we eliminated smoking from this group of smokers, 37. 9% of the CHD morbidity could be prevented. 12

Population Attributable Risk (PAR) o Recall that the AR quantifies the excess risk in the “exposed” group. o The PAR estimates the excess rate of disease in the “total study population” of exposed and nonexposed that is attributable to the exposure. 13

Uses of PAR o The PAR helps us to determine which exposures have the most relevance to the public’s health. o Can help guide the allocation of resources aimed at interventions to a whole population instead of intervening on high risk individuals (i. e. the exposed). 14

What do we need to calculate the PAR? o Incidence in the total population and incidence in the non-exposed group o Alternatively, we can use: n The incidence among exposed n The incidence among non-exposed n The proportion of the total population that is exposed 15

How to Calculate? PAR = IT – IU PAR = IRT – IRU Or PAR = CIT – CIU 16

PAR alternative calculation Alternatively, the PAR can be calculated as PAR = (AR)*Pe Where AR is the attributable risk and Pe is the proportion of exposed people in the population. 17

Recall Smoking and CHD example. . Exposure CHD yes CHD no Total Smokes 84 2, 916 3, 000 Doesn’t smoke 87 4, 913 5, 000 171 7, 829 8, 000 CIT = 171/8000 = 21. 4 per 1000 CIU = 87/5000= 17. 4 per 1000 PAR = CIT - CIU = 21. 4 - 17. 4= 4 per 1000 Interpretation: In this population, the excess occurrence of CHD attributable to smoking is 4 per 1, 000. 18

PAR% Similar to the AR%, the PAR% is the “proportion” of disease in the study population that is attributable to the exposure and could be eliminated if the exposure was eliminated. 19

How do we calculate PAR%? PAR% = (IT – IU)/ IT or PAR% = (CIT – CIU)/CIT PAR% = [Pe(RR-1)/((Pe(RR-1))+1)]*100 where RR is the relative risk of the exposure and disease (cohort study) and Pe is the proportion of exposed people in the population o Note: In a case-control study, when the disease is rare, the OR may be substituted for the RR. 20

Recall Smoking and CHD example. . Exposure Smokes Doesn’t smoke CHD yes 84 87 CHD no 2, 916 4, 913 Total 3, 000 5, 000 171 7, 829 8, 000 CIT = 171/8000 = 21. 4 per 1000 CIU = 87/5000= 17. 4 per 1000 PAR = CIT - CIU = 21. 4 - 17. 4= 4 per 1000 PAR% = (21. 4 - 17. 4)/21. 4= 18. 7% If smoking causes CHD, 18. 7% of the incidence of CHD in the total population can be eliminated if smoking were 21 eliminated

How do we use these measures? Figure 12 -3 Numbers of deaths attributed to major causes, United States, 2000. (Redrawn from Mokdad AH, Marks JS, Stroup DF, Gerberding JL: Actual causes of death in the United States, 2000. JAMA 291: 1238 -1245, 2004, with the correction from JAMA 293: 289, 2005. ) 22

Summary of AR calculations Incidence attributable to exposure (AR or PAR) Proportion of incidence attributable to exposure (AR% or PAR%) In Exposed Group In Total Population IE – I U IT – I U (IE – IU)/Ie (IT – IU)/IT 23