Abstract
In 1984, questionnaires were sent to the chief epidemiologist in all 50 states, Puerto Rico, and Washington, DC, with a 100 per cent response rate. There were substantial variations in case definitions of reportable diseases, criteria for counting as a case, and sources of surveillance. Laboratory reporting of any notifiable condition is mandated by 54 per cent of jurisdictions. These differences in ascertainment and case-counting practices constitute potential sources of error in national surveillance data on communicable diseases.