The PAR for a combination of risk factors is the proportion of the disease that can be attributed to any of the risk factors studied. The combined PAR is usually lower than the sum of individual PARs since a diseased case can simultaneously be attributed to more than one risk factor and so be counted twice.
When there is no multiplicative interaction (no departure from multiplicative scale), combined PAR can be manually calculated by this formula:
Read more about this topic: Attributable Risk
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