Weighted CUSUM procedures for surveillance of health events with varying population sizes

The CUSUM procedure has been popularly used for detecting a shift in the incidence rate of a rare health event. Many CUSUM methods are developed based on a Poisson model with a constant mean number of events. In practice, the expected number of events is likely to vary over time as the population size at risk is not constant but often grows over time. To account for the varying population size, this paper proposes the weighted CUSUM (WCUSUM) method. The simulation results show that the WCUSUM method is more efficient than the conventional CUSUM methods in detecting increases in the incidence rate, especially for small shifts.