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.
From those VICA queries where age adjustment is utilized, you can acquire age-adjusted rates using the U.S. 1940, 1970 or 2000 standard populations. The National Center for Health Statistics recommends that the U.S. 2000 standard population be used. 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.
The age-adjusted rate (AAR) is computed as 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, hospitalizations, etc.) 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 rate (ASR) per 100,000 population for each age group. That is, for each age group, ASR = events in age group ÷ estimated population of that age group x 100,000.
Each ASR 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 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 | 1940 Proportion |
1970 Proportion |
2000 Proportion |
| Under 1 year | 0.015343 | 0.017151 | 0.013818 |
| 1 - 4 years | 0.064718 | 0.067265 | 0.055317 |
| 5 - 14 years | 0.170355 | 0.200506 | 0.145565 |
| 15 - 24 years | 0.181677 | 0.174406 | 0.138646 |
| 25 - 34 years | 0.162066 | 0.122569 | 0.135573 |
| 35 - 44 years | 0.139237 | 0.113614 | 0.162613 |
| 45 - 54 years | 0.117811 | 0.114265 | 0.134834 |
| 55 - 64 years | 0.080294 | 0.091480 | 0.087247 |
| 65 - 74 years | 0.048426 | 0.061195 | 0.066037 |
| 75 - 84 years | 0.017303 | 0.030112 | 0.044842 |
| 85 and over | 0.002770 | 0.007435 | 0.015508 |
| All ages | 1.000000 | 1.000000 | 1.000000 |
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.