Concepts and Tests for Trend in Recurrent Event Processes

Interest in the presence and nature of trend arises frequently in science, public health, technology, and many other areas. In this ar- ticle we discuss the notion of trend in the context of recurrent event processes. We discuss different frameworks within which one can inves- tigate trend and consider various ways in which trends may be manifest. Tests for trend are discussed in detail and the utility of intensity-based models is emphasized for characterizing event processes and understand- ing trends. Simulation studies are conducted to study the effect of het- erogeneity in the investigation of trend. Data from a study of hospi- talization patterns in patients with affective disorder are analysed for illustration.