Population Attributable Risk Calculator

Enter exposure prevalence and relative risk to compute PAR and PAR% using the Levin formula.

Use the Population Attributable Risk Calculator

Input Parameters

Required fields use the Levin formula for PAR%.

Proportion of total population exposed to the risk factor (0–100).

%

Ratio of incidence in exposed vs. unexposed (must be > 0).

Optional: enter incidence rates to compute absolute PAR.

Cases per 100,000 person-years (or any consistent unit).

Cases per 100,000 in those NOT exposed.

Enter Pe and RR, then click Calculate to see results.

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Summary

The Population Attributable Risk (PAR) calculator quantifies how much of the disease burden in a total population is due to a specific exposure. It uses the Levin formula: PAR% = Pe(RR - 1) / [1 + Pe(RR - 1)] × 100, where Pe is the prevalence of exposure and RR is the relative risk. PAR% tells public health professionals what fraction of cases could theoretically be prevented if the exposure were eliminated from the population.

How it works

  1. Enter the prevalence of exposure (Pe) — the proportion of the population exposed to the risk factor (0–100%).
  2. Enter the Relative Risk (RR) — the ratio of disease incidence in exposed versus unexposed individuals.
  3. Optionally enter the incidence rate in the total population and the unexposed group to compute absolute PAR.
  4. Click Calculate to apply the Levin formula and see PAR% along with an interpretation.
  5. Use the Reset button to clear all fields and start a new calculation.

Use cases

  • Estimate the public health impact of eliminating a modifiable risk factor such as smoking or obesity.
  • Prioritize prevention programs by comparing PAR% across multiple exposures.
  • Support policy decisions by quantifying how many cases are attributable to a specific exposure.
  • Teach epidemiology students the Levin formula with an interactive, instant-feedback tool.
  • Conduct sensitivity analyses by adjusting Pe and RR to see how results change.
  • Compare attributable fractions across different populations or study designs.

Frequently Asked Questions

Last updated: 2026-06-04 · Reviewed by Nham Vu