Methods

Collection and Sources of Data

The data were taken from the December, 1999 analytic file and tabulated using SEER*Stat, a statistical software package distributed by the National Cancer Institute. The 1993-1997 population figures are provided by the U.S. Bureau of the Census through the Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute (NCI).

The New Jersey State Cancer Registry (NJSCR) follows 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. Note that, beginning with diagnoses in 1995, in situ cancers of the cervix are no longer collected, in accordance with the recommendations of NAACCR.

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 very few persons of unknown age and/or gender are excluded (see next section these numbers) as are out-of-state residents. 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

The percentage completeness of reporting was estimated by comparing 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 1991-1995 to similar ratios using NJ data. Using these standard formulae, it is possible for the estimation of completeness to be greater than 100%. For 1997, the completeness of case reporting was estimated as 99% when this report was prepared. Cancers among children (age 0-14) tend to be reported somewhat later than those among adults because many are diagnosed in out-of-state facilities that are not subject directly to New Jersey regulations. These cases are reported later under mutual agreements with New York, Pennsylvania, Delaware, Florida and North Carolina.

Other 1997 data quality indicators measured were:

  • Percent death certificate only cases: 2.1%
  • Percent of unresolved duplicates: 0.01%
  • Percent of unknown race: 0.8%
  • Percent of unknown county: 0.1%
  • Number of unknown age: 2
  • Number of unknown gender: 3

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 person-years of observation. 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.

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:

Formula
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).

The age-specific, race and gender specific population estimates for the state were provided by the SEER Program of NCI through an interagency agreement with the US Bureau of the Census. The age-specific rates are not shown in this report but are available from the NJSCR upon request.

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:

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).


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