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Estimating additive interactions from first principles

Tuesday, September 23, 2014 — Poster Session III

12:00 p.m. – 2:00 p.m.

FAES Academic Center




  • OA Panagiotou
  • S Wacholder


We discuss the theoretical basis of the additive interactions from first principles. Metrics of the impact of joint effects or comparisons of joint effects presented in units of absolute risk, such as Rothman’s I, can provide more meaningful quantitative measures of public-health impact of potential interventions that reduce exposures than “unit-less” metrics like ratios, and standardized metrics, like population attributable fraction. In particular, the venerable attributable community risk (ACR) metric can provide an estimate of the community impact of such interventions in units of absolute risk. A spreadsheet we provide demonstrates the calculations of the various metrics for hypothetical data. Using metrics of Rothman’s I and ACR, we show that positive interaction, or synergy, on the additive scale implies that the impact on risk reduction from a program that applies both interventions will be less than the sum of the impacts of the separate interventions.

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