Interpreting Incidence Data

Choice of Standard Population and Population Denominator

The U.S. Department of Health and Human Services’ policy for reporting death and disease rates was motivated by a need to standardize age-adjustment procedures across government agencies.1 2 Because of the aging U.S. population, the 2000 U.S. standard population gives more weight to older age categories than the 1940 and 1970 standard populations did.2

Because the incidence of cancer increases with age, using the 2000 U.S. standard population results in higher incidence rates for most cancers compared to incidence rates age-adjusted to the 1940 or 1970 standard populations.

The data in the Data Visualizations tool should not be compared with cancer incidence rates adjusted to different standard populations.

Incidence rates also are influenced by the choice of population denominators used in calculating these rates. Because some state health departments use customized projections of the state’s population when calculating incidence rates, the rates in the Data Visualizations tool may differ slightly from those published by individual states.

Registries’ Data Quality

Data quality is evaluated routinely by CDC’s National Program of Cancer Registries (NPCR) and the National Cancer Institute’s (NCI’s) Surveillance, Epidemiology, and End Results (SEER) Program.3 4 Comprehensive evaluation activities are conducted to find missing cases or to identify errors in the data. Although the cancer registries meet data quality criteria for all invasive sites combined, the completeness and quality of site-specific data may vary, making in-depth analyses critical to presenting reliable data. The observed rates may have been influenced by differences in the timeliness, completeness, and accuracy of the data from one registry to another, from one reporting period to another, or from one primary cancer site to another. In rare instances, a registry may identify a data quality issue after the file is submitted to CDC. In those instances, CDC will either suppress the identified segment or exclude the registry’s data from analytic products.

Reporting Time Intervals

Completeness and accuracy of the site-specific data also may be affected by the time interval allowed for reporting data to the two federal programs. The NPCR and SEER time interval for reporting data differ: for each submission year, NPCR allows a 23-month interval after the close of the diagnosis year and SEER allows a 22-month interval.

Reporting Delays

Delays in reporting cancer cases can affect timely and accurate calculation of cancer incidence rates.5 Cases are reported continuously to state and metropolitan-area cancer registries in accordance with statutory and contractual requirements. After the initial submission of the most recent year’s data to the federal funding agency, cancer registries revise and update their data on the basis of new information received. Therefore, some cancer cases likely will have been reported to state and metropolitan-area cancer registries after the registries submitted their data to CDC or NCI. For this reason, incidence rates and case counts reported directly by state or metropolitan-area cancer registries may differ from those that appear in the Data Visualizations tool. Reporting delays appear to be more common for cancers that usually are diagnosed and treated in non-hospital settings such as physicians’ offices (for example, early-stage prostate and breast cancer, melanoma of the skin). Efforts are underway to reduce reporting delays. Methods to adjust incidence rates for reporting delay were not applied to the data in this report.5

Continual Data Updates

Each year, central cancer registries submit an updated version of previous years’ data and data for a new diagnosis year to CDC and/or NCI. Federal agencies in turn update their cancer incidence statistics with each data submission and document the registries’ date of data submission whenever the data are published. These continual updates illustrate the dynamic nature of cancer surveillance and the attention to detail that is characteristic of cancer registries. Each year when United States Cancer Statistics data are released, we update data products with the most recent data submission.

Geographic Variation

Geographic variation in cancer incidence rates may result from regional differences in the exposure of the population to known or unknown risk factors.6 7 8 9 Differences may arise because of differences in sociodemographic characteristics of the population (age, race and ethnicity, geographic region, urban or rural residence), screening use, health-related behaviors (for example, tobacco use, diet, physical activity), exposure to cancer-causing agents, or factors associated with the registries’ operations (completeness, timeliness, specificity in coding cancer sites). Cancer researchers are investigating variability associated with known factors that may affect cancer rates and risks by using model-based statistical techniques and other approaches for surveillance research. Differences in registry operations are being evaluated to ensure consistency and quality in reporting data.


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