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|Let ω be the number of deaths attributable to a risk factor (e.g. overweight and obese); and ωi be the number of deaths attributable to a specific exposure category i (e.g. overweight) of the risk factor.|
|Using the notations in Table Table22,|
|Using the notations of Allison et al, equation 11 can be expressed as|
|Using the notations of Mokdad et al, and adding Pq (fraction of population underweight) and RRq (relative risk for underweight), equation 11 becomes|
Eight steps to estimate ω:
Step 1: Obtain M, total no. of deaths in the population during 1 y (from death records).
Step 2: Obtain f0 [or P(R) or P0], percentage of individuals in the population nonexposed (e.g. normal weight; never smokers) (from health surveys).
Step 3: Obtain fi [or P(Oi) or Pi], percentage in separate exposure categories of the risk factor (e.g. overweight or obese; current occasional smokers or current daily smokers) (from health surveys).
Step 4: Obtain fq [or P(Q) or Pq], percentage in a neutral category (e.g. underweight; former smokers) (from health surveys).
Step 5: Obtain Ri [or RRi], relative risk of death for each separate exposure category (e.g. former smokers, current smokers) relative to none (e.g. never smokers) (from prospective cohort studies). When Ri is not available and when the event (e.g. death) is rare, relative risk can be approximated by HR (ie, h and q).
Step 6: Calculate PAFi, population attributable fraction, using equation 11.
Step 7: Calculate ωi using equation T5.
Step 8: Calculate ω using equation T4.