Using reference values to define disease based on the lower limit of normal biased the population attributable fraction, but not the population excess risk: the example of chronic airflow obstruction

The impact of disease on population health is most commonly estimated by the population attributable fraction (PAF). This measurement, an estimate of the proportion of the disease in the population that is attributable to the exposure, is sensitive to the way that the disease is defined. When disease is defined from reference values that are derived from the distribution of values in the "normal" population the PAF is difficult to interpret. Using measures of chronic airflow obstruction as an example, we demonstrate that where normality is defined by centiles (or fractiles) of values in a "normal" population, PAF is strongly influenced by which centile is selected to define normality.

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