Average age at first occurrence as an alternative occurrence parameter in epidemiology.

Background: Current epidemiological methods focus mostly on incidence rates and their ratios as measures of occurrence and effect. Incidence rates and rate ratios can be hard to interpret when the outcome disease is common.

Methods: The possibilities of using an alternative measure, average age at first occurrence of the disease, are discussed.

Results: Methods for the analysis of average age at first occurrence are illustrated in a study of occupation and disability pension. We estimate that the average age at pension among crane drivers is only about 0.8 that of maintenance workers, regardless of the method, although the confidence intervals depend on the method.

Conclusions: Average age at first occurrence is easier to interpret than the incidence rate for a very common disease. Because estimates are not readily available in software packages, we encourage further development of packaged software for this measure.