Harmonic Analysis Applied to Seasonal Variations in Sickness Absence

SUMMARY Harmonic analysis is used to study variations in the weekly incidence of new spells of sickness absence over a period of several years. Such an approach, using the non-homogeneous Poisson process, allows weekly fluctuations to be separated into random, seasonal and non-seasonal components. The model is illustrated by data from one industrial organization, the medical diagnosis and other factors related to sickness absence being considered. The methods developed in this study are applicable to any essentially cyclic phenomenon represented by a series of equally spaced counts.