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A basic measure of disease frequency is a rate, which takes into account the number of cases or deaths and the population size. For example, if a cancer incidence rate is 500 per 100,000, it means that 500 new cases of cancer were diagnosed for every 100,000 people.
Crude, age-specific, and age-adjusted rates with corresponding 95% confidence intervals are presented in the tables. Age-adjusted rates and corresponding 95% confidence intervals are presented in the graphs. A description of each is provided below.
- A crude rate is the total number of cases or deaths divided by the total population and multiplied by 100,000 (for cancers by primary site) or by 1 million (for International Classification of Childhood Cancer [ICCC] groupings of childhood cancers).
- Crude rates are influenced by the underlying age distribution of the state’s population. If two states have the same age-adjusted rates, the state with the relatively older population (as demonstrated by having a higher mean age) will have higher crude incidence rates because incidence or death rates for most cancers increase with increasing age.
- Crude rates are helpful in determining the needs for services for a given population, relative to another population, regardless of size.
- An age-specific rate is the number of cases or deaths in a specified age category divided by the population in the specified age category multiplied by 100,000 (for cancer by primary site) or by 1 million (for ICCC grouping of childhood cancers).
- The occurrence of many cancers increases with age, as does cancer mortality. The age distribution of a population (the number of people in particular age categories) can change over time and can be different in different geographic areas.
- The use of age-adjusted rates permits a valid comparison between, for example, one year’s rates and another year’s or between one geographic area’s rates and another area’s.
- Age-adjusting the rates ensures that the differences in incidence or deaths from one year to another or from one geographic area to another are not due to differences in the age distribution to the populations being compared.
- An age-adjusted rate calculated using the method is described on Incidence and Death Rates.
95% Confidence Interval
- Confidence intervals reflect the range of variation in the estimation of the cancer rates.
- The width of a confidence interval depends on the amount of variability in the data. Sources of variability include the underlying occurrence of cancer as well as uncertainty about when cancer is detected and diagnosed, when a death from cancer occurs, and when the data about the cancer are sent to the registry or the state health department.
- A 95% confidence interval calculated using the method is described on Confidence Intervals.