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Notice to Readers: Interpretation of Provisional Data Presented in
Morbidity and Mortality Weekly Report Tables
Periodically, readers of MMWR interpret provisional data about the incidence of notifiable infectious disease presented
in MMWR incorrectly as finalized incidence data. This occurred recently in a media report that included a misinterpretation
of the provisional acquired immunodeficiency syndrome (AIDS) data reported to the National Notifiable Diseases
Surveillance System (NNDSS) and presented in
MMWR Table II (1). Although the characteristics of the
MMWR morbidity data (provisional versus finalized) are footnoted, this notice reminds readers about the different characteristics of provisional
and finalized data.
Provisional NNDSS data are a running count of infectious disease (including AIDS) cases that have not been adjusted
for variations in reporting procedures across different states
or for delays in reporting. In addition, data in
MMWR Table II reflect the date cases were reported to CDC rather than the date persons were actually diagnosed with the notifiable
condition. Morbidity data adjusted to reflect the number of notifiable disease cases diagnosed during a given year reflect
disease incidence trends more accurately than a comparison of end-of-year provisional data (e.g., week 52 data for a current
year) with finalized data. For example, although 46,143 AIDS cases were reported to CDC in 1999, an estimated 41,850
persons were diagnosed with AIDS; duplicates, errors, or persons who had AIDS diagnosed earlier than 1999 accounted for
the remainder of cases (2).
To provide timely NNDSS data, state and territorial health departments voluntarily report notifiable disease incidence
data to CDC as soon as they become aware of these cases. These provisional data are published each week in
MMWR to disseminate the most current national information about infectious diseases to public health officials and
health-care providers so they can initiate prevention and control activities. These data include case reports considered "suspect"
or "probable" for surveillance purposes in addition to those considered
"confirmed"(3). As a result, provisional data are
subject to change based on the outcome of further case investigation. As part of the process for finalizing surveillance
data, provisional NNDSS data, including AIDS incidence data, also are adjusted for variations in reporting practices and
delayed
reporting. CDC amends and edits provisional data periodically throughout the year as updates are reported by states.
The process for finalizing case counts can take several months to complete after the end of the year. Cumulative (i.e.,
year-to-date) incidence data from the previous year also are presented in Table II as a crude method to identify aberrations or
discrepancies in reported disease incidence data---whether because of disease incidence or reporting artifacts. Because of differences
in timing of reports, the source of the data, and the use of different case definitions, NNDSS data published in the
weekly edition of MMWR also might differ from data published in other CDC surveillance reports.
CDC. Letter dated January 22, 2002 to the Editor, Knight Ridder Washington Bureau, from Harold W. Jaffe, MD, Acting Director of the
National Center for HIV, STD, and TB Prevention.
Use of trade names and commercial sources is for identification only and does not imply endorsement by the U.S. Department of
Health and Human Services.References to non-CDC sites on the Internet are
provided as a service to MMWR readers and do not constitute or imply
endorsement of these organizations or their programs by CDC or the U.S.
Department of Health and Human Services. CDC is not responsible for the content
of pages found at these sites. URL addresses listed in MMWR were current as of
the date of publication.
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Users should not rely on this HTML document, but are referred to the electronic PDF version and/or
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