Childhood Cancer
Appendix A
Data and Statistical Methods

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The data and statistical methods used throughout the report are described below.


The data for this report come from several sources. The New Jersey State Cancer Registry provides information on each cancer case. The quality of the registry data has continued to improve since casefinding began in 1979, and in 1998 the registry was certified as meeting the Gold Standard on quality, completeness and timeliness, the highest standard of quality among central cancer registries. The proportion of all cases that come from death certificates only is one measure of data quality. Fifteen percent of cases were from death certificates only in 1979, and by 1995, just 2.8 percent of cases were from death certificates only. Unresolved duplicates, another data quality measure, comprised 0.10 percent of all 1995 cases. The registry is currently estimated to be over 98 percent complete.

The data for mortality rates in New Jersey are produced by the Vital Statistics division of the New Jersey Department of Health. Deaths are attributed to causes based on the underlying cause of death indicated on each death certificate.

The population counts that serve as denominators for the New Jersey rates presented in this report are provided by the Surveillance, Epidemiology and End Results (SEER) program of the National Cancer Institute (NCI) through an interagency agreement with the United States Bureau of the Census. These population estimates are also subject to error, but this source of error is not accounted for in the rates presented in this report.

National-level cancer incidence estimates come from the SEER program and are used in comparisons with the State-level data. The SEER data are derived from approximately 10 percent of the United States population. Although both the state and national rates presented here are adjusted for differences in age distribution, there are other unaccounted factors such as race, access to health care, and urban/rural status which may limit our ability to make meaningful comparisons.

National-level cancer mortality rates come from the National Center for Health Statistics, Vital Statistics program. These data are based on death certificates from all 50 states.

Statistical Methods

The cancer incidence rate is the number of new cancer cases detected over a specified time-period in a given population. Typically, the cancer incidence rate is expressed as the number of new cases per 100,000 people. Mortality rates are computed and expressed similarly. In this report, rates are expressed per 100,000 children.

Age is one of the most important determinants of cancer. Incidence and mortality rates are frequently calculated separately for specific age groups. These rates are referred to as age-specific rates. The age-specific rate for a time period of length t is calculated as follows:

where ra = the age-specific rate for age-group a,

na = the number of events (cancer diagnoses or deaths, for example) in age-group a during the time period,

t = the length of time in years, and

Pa = average size of the population in age-group a during time t (mid-year population or average of the mid-year populations).

Multiplying ra by 100,000 expresses the rate as the number of cases per 100,000 persons.

When comparing rates across different population subgroups, e.g. geographic region, it is important to account for different age distributions. We calculate an age-adjusted rate using a weighted-average of the age-specific rates. This method of age adjustment is known as direct age-standardization. The age-adjusted rate is obtained by using the age distribution of a standard population as the weights:

whereR = the age-adjusted rate,

ra = the age-specific rate for age-group a, and

Std.Pa = the number of people in age group a of the standard population.

Multiplying the age-adjusted rate by 100,000 expresses it as the number of cases per 100,000 persons. Age-adjusted childhood cancer incidence and mortality rates are based solely on age groups under age 15.

The standard population used for age adjustment throughout this report is the 1970 United States population. This is the traditional standard population used in much of the published cancer incidence data and provides a common "yard-stick" that can be used to distinguish meaningful differences in cancer incidence and cancer mortality.

Summary of Data and Statistical Methods

The cancer incidence rate is defined as the number of new cases per 100,000 persons. Similarly, the mortality rate is the number of deaths per 100,000 persons. In this report, all rates are

expressed per 100,000 children. Small numerators and denominators as well as small changes in the numerator may result in a large amount of variability in incidence and mortality rates.

Caution is advised in the interpretation of rates and comparisons because sources of variability have not been accounted for. Unmeasured variation includes measurement error and sampling error. Confounding factors, which may obscure differences in rates, have also not been incorporated into estimates presented in this report.

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