The idea behind this is to assess the difference in the outcome from the presence in the predictor variable versus the absence of the predictor variable. In other words, the formal equation is the incidence of disease in the presence of the exposure minus the incidence of disease without the exposure. Then only you can determine the amount actually attributable to the exposure.
Population Attributable Risk (PAR) is the proportion of the incidence of a disease in the population (exposed and nonexposed) that is due to exposure. PAR is the difference between the risk in the total population and that in unexposed subjects. It is the incidence of a disease in the population that would be eliminated if exposure were eliminated
Population attributable risk: (a/(a+c))(RR-1/RR)
Another way to look at it:
What amount of the risk of disease in a population is attributable to a specific exposure?" (as opposed to baseline population risk)
total population incidence of disease - incidence of disease amongst unexposed
= [(a+c)/(a+b+c+d) - (c/c+d)]
unit is per person
e.g., if your PAR for high blood pressure on heart disease is 0.004, the risk of heart disease for the total population that is likely due to high blood pressure is 4 cases per 1,000 people
if obesity was eliminated from the population, 4 cases of cardiac disease per every 1,000 people would be eliminated
Population attributable risk percent (PAR%)
"What percent of disease cases amongst a population can be attributed to a specific exposure?" (as opposed to baseline population risk)
[(total population incidence of disease - incidence of disease amongst unexposed)/total population incidence of disease] * 100
= [(a+c)/(a+b+c+d) - (c/c+d)]/[(a+c)/(a+b+c+d)] * 100
World Health Organization defines this as the proportional reduction in population disease or mortality would occur if exposure to a risk factor were reduced to an alternative ideal exposure scenario.
This is also an example of how we can use technology to address equity and even visualizing individuals from historically marginalized groups to be in professional settings as a senior epidemiologist.