COMPREHENSIVE SPECTRAL-ANALYSIS OF TIME-SERIES DATA OF RECURRENT EPIDEMICS

A comprehensive spectral analysis of data from the surveillance of inf ectious diseases in Japan on the weekly incidences of measles, chicken pox, mumps and rubella was carried out using a computing program known as ''MemCalc.'' Each power spectral density (PSD) indicated exponenti al decay until it leveled off at a limit determined by the accuracy of the present data. Many spectral lines observed in the PSD could be fu lly assigned using linear combinations of a few Fundamental periods. T he optimum least squares fitting curve calculated using these fundamen tal periods essentially reproduced the underlying variation of disease data and an extension of the curve could be used to predict the incid ence of the diseases. The effect of mass vaccinations on measles was q uantitatively estimated by analyzing the monthly data from Statistics on Communicable Disease. The periodic modes for seasonal oscillations were also estimated. Some discussions including the chaotic dynamics o f data on the outbreak of diseases are presented. The method of analys is was confirmed to be a highly effective in elucidating temporal vari ations of disease data of short data length.