Age-adjusted Incidence Rates

Age-adjusted incidence rates published within this report were adjusted using the direct method and standardized to the age distribution of the U.S. 1970 population (see Appendix B for the 1970 U.S. standard population). The rate represents the average number of new cases diagnosed annually per 100,000 persons. Age adjustment allows rates from one geographic area to be compared with rates from another geographic area that may have differences in age distributions. Any observed differences in age-adjusted incidence rates between populations are not due to differing age structures.

The computation of rates requires reliable estimates of the population at risk by five-year age groups and gender during the time period being studied. Population estimates used in this report were extrapolated from the U.S. Bureau of the Census 1990 population figures and the Idaho Center for Vital Statistics and Health Policy’s 1995 postcensal population estimates (see Appendix C).7

In conformity with the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) Program guidelines the incidence rates excludes the following:

  • in-situ cases (373 cases)
  • basal and squamous cell skin cancers (117 cases)
  • cases with unknown age (0 cases)
  • cases with unknown gender (3 cases)

Of the total number of cases for 1995 (4,783) a total of 4,290 cases are used for calculating age-adjusted incidence rates. Of the 4,290 cases 2,215 are males and 2,075 are females.

Age-specific Incidence Rates

Age-specific rates are calculated by dividing the number of cases for a given age-group by the total population of that age group and are expressed as an average annual rate per 100,000 population by age group. Age-specific rates also excludes the same types of cases as those excluded from age-adjusted incidence rates.

Observed vs. Expected Number of Cases

The expected number of cases were calculated using the direct method of age-adjustment combining the population of the health district under consideration with that of the rest of the state of Idaho. The observed and expected numbers exclude in-situ cases, basal and squamous cell skin cancers, as well as cases with unknown age or sex. Comparison of the observed to the expected (standardized incidence ratio) is statistically significant at p £ 0.05. The asterisked (*) comparisons are those that are significantly different statistically. Statistical significance does not necessarily imply that concern is warranted since differences can occur as a result of multiple factors other than chance alone.

Risk and Associated Factors

The “risk and associated factors” subsections in Section I were developed from extracts of the 1993 annual report of the Washington State Cancer Registry and the “American Cancer Society Textbook of Clinical Oncology”. 8-9 Mean/Median/Mode

Measures of central tendency are helpful to describe a group of individual values in a simple and concise manner:

Mean: also known as the arithmetic average, is the sum of all observations divided by the number of observations.
Median: is the value in terms of magnitude, which divides the distribution of observations into two equal parts.
Mode: is the value which occurs most frequently in a group of observed values.

Confidence Intervals

A range of values, determined by the degree of variability in the data, within which the true value is thought to lie.

Cancer Case Definition

A “cancer case” is defined as a primary cancer site (where the cancer started) not a metastatic cancer site (where the cancer spread to). Since an individual can have more than one primary cancer site during their lifetime, the number of incident cancer cases are greater than the number of persons who are diagnosed with cancer.

Limitations to Data Interpretation and Comparison

Rates based on population estimates: State and county population estimates have been revised by the Bureau of the Census since the production of the estimates used for this report.

Rate comparisons: Age-adjusted incidence rates and age-specific rates based on small numbers of cases (fewer than 10) may be unstable. Rate comparisons between geographic areas (counties, health districts, or states) should consider demographic differences among their populations. Interpretations without consideration of these factors may be misleading or inaccurate.

Racial misclassification: Many source documents used to report cancer do not specify race of the patient. When race is not specified the case is coded as Caucasian. This can result in bias.

Standard Site Analyses Categories

To facilitate interpretation of data and comparisons across registries, CDRI uses standardized groupings of standard site analysis categories. These groupings are consistent with the National Cancer Institute’s SEER Program and are adopted by NAACCR. 4,5 Most neoplasms are grouped by the organ where they occur. Neoplasms of the lymphatic, hematopoietic, as well as the reticuloendothelial system are grouped by their histologies (leukemias, lymphomas, etc), and not by the anatomical site where they occurred (see Appendix A for groupings of codes).

Stage at Time of Diagnosis

Staging measures the extent of disease at the time of initial diagnosis. Summary staging attempts to group cases with similar prognoses into categories of:

  • in-situ (non-invasive)
  • localized (cancer confined to the primary site)
  • regional (direct extensive of tumor to adjacent organs, and/or lymph nodes
  • distant (metastasis to tissues or lymph nodes remote from the primary site)
  • unknown