What is Population Attributable Risk (PAR)?

PAR is the difference between the incidence rate in the total population and the incidence rate in the unexposed group. It represents the excess disease burden in the population that is attributable to the exposure.

What is PAR% and how is it different from PAR?

PAR% (also called Population Attributable Fraction or PAF) expresses PAR as a percentage of the total population incidence. While PAR is an absolute rate difference, PAR% is a relative measure showing what fraction of all cases could be eliminated if the exposure were removed.

What is the Levin formula?

The Levin formula calculates PAR% directly from exposure prevalence (Pe) and relative risk (RR): PAR% = Pe(RR - 1) / [1 + Pe(RR - 1)] × 100. It is valid when the exposure prevalence is measured in the total population.

What does a PAR% of 40% mean?

A PAR% of 40% means that 40% of all cases in the population are attributable to that exposure. Theoretically, eliminating the exposure would reduce the total disease incidence by 40%.

Can PAR% exceed 100%?

No. PAR% is bounded between 0% and 100%. A result near 100% would mean almost all cases in the population are due to the exposure, which is only plausible when both Pe and RR are very high.

When is this calculator NOT appropriate to use?

The Levin formula assumes the association between exposure and disease is causal, that no confounding is present, and that the exposure prevalence is measured in the total population (not just cases). Violations of these assumptions will produce misleading PAR% estimates.

Population Attributable Risk Calculator

Name: Population Attributable Risk Calculator
Availability: InStock
Author: Nham Vu

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

Input Parameters

Required fields use the Levin formula for PAR%.

Exposure Prevalence, Pe *

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

Relative Risk, RR *

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

Optional: enter incidence rates to compute absolute PAR.

Incidence — Total Population, I_t

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

Incidence — Unexposed Group, I_u

Cases per 100,000 in those NOT exposed.

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

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Summary

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

How it works

Enter the prevalence of exposure (Pe) — the proportion of the population exposed to the risk factor (0–100%).
Enter the Relative Risk (RR) — the ratio of disease incidence in exposed versus unexposed individuals.
Optionally enter the incidence rate in the total population and the exposed group to compute absolute PAR.
Click Calculate to apply the Levin formula and see PAR% along with an interpretation.
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: 2026-06-09 · Reviewed by Nham Vu