Determination of the numbers of monitoring medical institutions necessary for estimating incidence rates in the surveillance of infectious diseases in Japan

Our purpose was to determine the number of monitoring stations (medical institutions) necessary for estimating incidence rates in the surveillance system of infectious diseases in Japan. Infectious diseases were selected by the type of monitoring stations: 15 diseases in pediatrics stations, influenza in influenza stations, 3 diseases in ophthalmology stations and 5 diseases in the stations of sexually transmitted diseases (STD). For each type of monitoring station, 5 cases of the number of monitoring stations in each health center, including the number determined from presently established standards and the actual number in 1997, were given. It was assumed that monitoring stations were randomly selected among medical institutions in health centers. For each infectious disease, each case and each type of monitoring station, standard error rates of estimated numbers of incidence cases in the whole country were calculated in 1993-1997 using the data of the surveillance of infectious diseases. Among 5 cases of monitoring stations, the case satisfied the condition that those standard error rates were lower than the critical values, was selected. The critical values were 5% in pediatrics and influenza stations, and 10% in ophthalmology and STD stations. The numbers of monitoring stations in the selected cases were 3,000 in pediatrics stations, 5,000 in influenza stations (including all pediatrics stations), 605 in ophthalmology stations and 900 in STD stations.