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 figures used in this report were obtained from U.S. Bureau of the Census estimates of the population of counties by age and gender, 1990-1996 (see Appendix C).7
In conformity with the National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) Program guidelines the incidence rates excluded the following:
- in-situ cases, except bladder (268 cases)
- basal and squamous cell skin cancers (105 cases)
- cases with unknown age (1 case)
- cases with unknown gender (0 cases)
Of the total number of cases for 1996 (4,780) a total of 4,406 cases were used for calculating age-adjusted incidence rates. Of the 4,406 cases, 2,322 occurred among males and 2,084 occurred among 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 exclude the same types of cases that are excluded from age-adjusted incidence rates.
Observed vs. Expected Number of Cases
The expected numbers of cases were calculated using the indirect method of age-adjustment. For each health district, the expected numbers of cases were calculated using rates for the remainder of Idaho. The observed and expected numbers exclude in-situ cases, basal and squamous cell skin cancers, and cases with unknown age or sex. Statistically significant differences between observed and expected cases (standardized incidence ratios) were marked (+) for p£ 0.05 and (*) for p£ 0.01. Statistical significance does not necessarily imply that concern is warranted since differences can occur as a result of multiple factors.
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 middle value when the observations are ranked in order from the smallest to the largest.
Mode: is the value which occurs most frequently in a group of observed values.
An estimated range of values within which the true population value lies with given probability.
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 figures are estimates, and errors in the estimates will also impact the rates.
Rate comparisons: Age-adjusted incidence rates and age-specific rates based on small numbers of cases (fewer than 10) may be unstable. In comparing rates among geographic areas (counties, health districts, or states), factors such as the absolute numbers of cases and differences in demographics should be considered. 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)