Age-Adjusted Rate Definitions

Crude Rates

Rates allow for fairer comparisons between geographies with different population totals. Crude rates also account for the total burden of a health outcome to a community. This statistic is calculated as the number of events (numerator) divided by the population at risk (denominator). The population at risk is “a term applied to all those whom an event could have happened, whether it did or not.” For many health statistics, the denominator is simply the population total.

Age-Specific Rates

Sometimes, a researcher may be interested in the relative health burden of a condition to only a subset of the population. If this subset is based on a specific age group, the statistic is often referred to as an age-specific rate. Much like a crude rate, this statistic measures the number of events (numerator) divided by the population at risk (denominator), which in the case of age-specific rates would only include the total number of individuals within a certain age group.

Age-Adjusted Rates

Age adjusting rates is a way to make fairer comparisons between groups with different age distributions. For example, a county having a higher percentage of elderly people may have a higher rate of death or hospitalization than a county with a younger population, merely because the elderly are more likely to die or be hospitalized. (The same distortion can happen when comparing races, genders, or time periods.) Age adjustment can make the different groups more comparable.

A "standard" population distribution is used to adjust death and hospitalization rates. The age-adjusted rates are rates that would have existed if the population under study had the same age distribution as the "standard" population. Therefore, they are summary measures adjusted for differences in age distributions.

The National Center for Health Statistics recommends that the U.S. 2000 standard population be used when calculating age-adjusted rates. Users of Missouri Information for Community Assessment (MICA) have the option of selecting age-adjusted rates based on the U.S. 1940, 1970 or 2000 standard populations when generating tables where age-adjustment is utilized. The National Center for Health Statistics recommends that the U.S. 2000 standard population be used when calculating age-adjusted rates. However, if you compare rates from different sources, it is very important that you use the same standard population on both sides of your comparison. It is not legitimate to compare adjusted rates which use different standard populations.

Age-adjusted rates in the Community Data Profiles use the U.S. 2000 standard population.

Computation

An example of the computation of an age-adjusted death rate (AADR) follows:

The rate in the area of study (e.g., county, state) is computed for each age group noted in the table below by dividing the number of events (deaths) in that age group by the estimated population of the same age group in that area and then multiplying by a constant of 100,000. This results in an age-specific death rate (ASDR) per 100,000 population for each age group. That is, for each age group, ASDR = deaths in age group ÷ estimated population of that age group × 100,000.

Each ASDR is then multiplied by the proportion of the standard population (see table below) that same age group. The age-specific results are summed to get the age-adjusted death rate for the area of study. The formula is:

This is called the direct method of standardization. A given area's age-specific rate (overall or for a given cause) is applied to the U.S. standard population.

The following are the U.S. standard population distributions:

Age-adjusted rates published elsewhere (e.g., in the annual Missouri Vital Statistics) may be slightly different from those found in the MICAs or Community Data Profiles, due to updating of population estimates for years between decennial Censuses. The constant or "per population" number used for the age-adjusted rates may vary, depending on the type of event. For example, the age-adjusted rates for deaths are per 100,000 population. However, age-adjusted rates for hospitalizations and procedures are per 10,000 population and age-adjusted rates for emergency department visits are per 1,000 population.

References

Population at risk definition acquired from:
Austin, DF, Werner SB. Epidemiology for the health sciences: A primer on epidemiologic concepts and their uses. Springfield, IL: Charles C. Thomas; 1974.

1940 and 2000 U.S. standard populations acquired from:
Anderson RN, Rosenberg HM. Age Standardization of Death Rates: Implementation of the Year 2000 Standard. National Vital Statistics Reports; Vol. 47 No. 3. Hyattsville, Maryland: National Center for Health Statistics. 1998.

1970 U.S. standard population acquired from:
Devessa S, Grauman DJ, Blot WJ, Pennell GA, Hoover RN, Fraumeni JF Jr. Atlas of Cancer Mortality in the United States, 1950-94. Bethesda, Maryland: National Cancer Institute, 1999. NIH publication 99-4564.