Decomposing Time Series

Abstract

Many common diseases, such as the flu and cardiovascular disease, increase markedly in winter and dip in summer. These seasonal patterns have been part of life for millennia and were first noted in ancient Greece by both Hip- pocrates and Herodotus. Recent interest has fo- cused on climate change, and the concern that seasons will become more extreme with harsher winter and summer weather. We describe a set of R functions designed to model seasonal pat- terns in disease. We illustrate some simple de- scriptive and graphical methods, a more com- plex method that is able to model non-stationary patterns, and the case-crossover to control for seasonal confounding.