Cancer Incidence and Mortality in New Jersey, 1994 -1998

Methods

Collection and Sources of Data

The incidence data were taken from the November 2000 analytic file and tabulated using SEER*Stat, a statistical software package distributed by the National Cancer Institute. The 1994-1998 population estimates are provided by the U.S. Bureau of the Census.

The NJSCR follows the guidelines and standard practices of the SEER Program in determining multiple primary cancers for an individual. Statistics for in situ cancers of the breast and cervix are presented, but not included in the statistics for cancers of all sites.

An individual may develop more than one type of cancer within a given year. Therefore, the number of cancer cases is greater than the number of cancer patients. The out-of-state residents and cases with unknown age and/or gender are excluded. Race-specific information is not shown for persons of non-white or non-black races (including unknown race), but this information is included in the "all races" data.

Estimation of Completeness and Other Data Quality Indicators

Completeness of reporting was estimated by comparing New Jersey and U.S. incidence to mortality ratios for whites, standardized for age, gender, and cancer site. The data used to generate these ratios were the cancer incidence rates for all SEER registries combined and the U.S. mortality rates for years 1992-1996. Using these standard formulae, it is possible for the estimation of completeness to be greater than 100%. For 1998, the completeness of case reporting was estimated as 105.5% when this report was prepared. In contrast, the 1997 rates for the comparable incidence report for the years 1993-1997, which we issued in March of 2000, was calculated to be 99% at that time. Cancers among children (age 0-14) tend to be reported somewhat later than those among adults because many pediatric cases are diagnosed in out-of-state facilities that are not subject directly to New Jersey reporting regulations. Out-of-state cases are reported under mutual agreements with New York, Pennsylvania, Delaware, Florida, and Maryland.


Other 1998 data quality indicators measured were as follows:

Incidence:

Percent death-certificate-only cases: 1.8 %
Percent of unresolved duplicates: 0.09%
Percent of unknown race: 0.3%
Number of unknown age: 5
Number of unknown gender: 1

Mortality:

Number of unknown race: 4
Number of unknown age: 4
Number of unknown gender: 0

Calculation of Rates

A cancer incidence rate is defined as the number of new cases of cancer detected during a specified time period in a specified population. These rates are most commonly expressed as cases per 100,000 population. Cancer occurs at different rates in different age groups, and population subgroups defined by gender and race have different age distributions. Therefore, before a valid comparison can be made between rates, it is necessary not only to adjust the rates by age but also to standardize the rates to the age distribution of a standard population. In this report, the 1970 US Standard Million population was used. Mortality rates are similarly defined and calculated. Records that were missing gender, age, or race could not be included in the rates presented in this report. Since the number of records so affected are very small, the rates are virtually unaffected by the non-inclusion of these records.

The first step in this procedure was to determine the age-specific rates. For each age-group for a given time interval (within each race-gender group, for the entire state), the following formula was applied:


where:
ra = the age-specific rate for age-group a,
na = the number of events (cancer diagnoses) in the age-group during the time interval,
t = the length of the time interval (in years), and
Pa = average size of the population in the age-group during the time interval (mid- year population or average of mid-year population sizes).

In order to determine the age-adjusted and standardized rate, a weighted average of the age-specific rates was calculated, using the age distribution of the standard population to derive the age-specific weighting factors (Rothman, 1985). This is the technique of direct standardization, which uses the following formula:


where:
R = the age-adjusted rate
ra = the age-specific rate for age-group a, and
Std.Pa = the size of the standard population in each age-group a.

While age adjustment and standardization facilitates the comparison of rates among different populations, there can be important age-specific differences in disease occurrence, which are not apparent in comparisons of the age-adjusted rates (Breslow and Day, 1987).

Analogous definitions and calculations apply for cancer mortality rates.


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