Age-Adjusted Rate

Age-adjusting a rate 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 we compare 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 been distributed by age the same way as in the "standard" population. Therefore, they are summary measures adjusted for differences in age distributions.

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 population, when generating tables for data 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 rate for deaths (AADR) follows:

The rate in the area of study (e.g., county, state) is computed for each age group noted in the table below. The number of events (deaths) in that age group in the area is divided by the estimated population of the same age group/area and then multiplied by 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 was in that age group. Then the age-specific results are added up to get the age-adjusted death rate for the area of study. The formula is:

AADR = Summation of (ASDR X standard proportion)

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 Censuses. The "per population" number used for the age-adjusted rate may vary, depending on the type of event. For example, the age-adjusted rates for deaths are per 100,000 population. However the age-adjusted rates for hospitalizations and procedures are per 10,000 population and the age-adjusted rates for emergency department visits are per 1,000 population.

The use of different standard populations can also affect general trends in total mortality and cause of death and differences in mortality by race and gender. For more information on this topic see: "Effects of Changing from the 1940 to the Year 2000 Standard Population for Age-Adjusted Death Rates in Missouri": Missouri Monthly Vital Statistics, Feb 2000, Vol.33 No. 12.

References:
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.