Update: changes in notifiable disease surveillance data--United States, 1992-1993.

Since April 1990, CDC has graphically presented changes in reported cases of 14 notifiable infectious diseases reported through the National Notifiable Diseases Surveillance System (NNDSS). Figure I of each issue of MMWR displays on a log scale the ratio of the number of cases reported in the most recently ended 4-week period to that of the mean of the number of cases reported in 15 historical 4-week periods. During 1992-1993, Figure I has indicated a decline in incidence in selected reportable diseases (e.g., measles, mumps, and rubella). Declines in the incidence of these diseases could represent either true declines or changes in notifiable disease surveillance (e.g., a lower percentage of all cases was reported than previously). During June-August, 1993, CDC evaluated the surveillance system to determine the reasons for the declines. This report summarizes the results of the evaluation.