# Year 2000 Standard Million Population for the United States — Age-Adjusted Rates

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Age-adjusted death rates are weighted averages of the age-specific death rates, where the weights represent a fixed population by age. They are used to compare relative mortality risk among groups and over time. An age-adjusted rate represents the rate that would have existed had the age-specific rates of the particular year prevailed in a population whose age distribution was the same as that of the fixed population. Age-adjusted rates should be viewed as relative indexes rather than as direct or actual measures of mortality risk.

The year “2000 U.S. standard” is the default population selection for the calculation of age-adjusted rates. However, you can select other standard populations, or select specific population criteria to determine the age distribution ratios. See Frequently Asked Questions about Death Rates for more information.

The rates of almost all causes of death vary by age. Age adjustment is a technique for “removing” the effects of age from crude rates, so as to allow meaningful comparisons across populations with different underlying age structures. For example, comparing the crude rate of heart disease in Florida to that of California is misleading, because the relatively older population in Florida will lead to a higher crude death rate, even if the age-specific rates of heart disease in Florida and California are the same. For such a comparison, age-adjusted rates are preferable. Age-adjusted rates should be viewed as relative indexes rather than as direct or actual measures of mortality risk.

The National Center for Health Statistics (NCHS) age-adjusts death rates using the direct method. That is, by applying age-specific death rates (Ri) to the U.S. standard population age distribution.

R’ = S i ( Psi / Ps ) R i

where Psi is the standard population for age group i and Ps is the total U.S. standard population (all ages combined).

In the direct method, a standard age distribution is chosen and the age-specific death rates are weighted according to the standard. A reasonable choice for the standard is the U.S. total population (all races, both genders) for the year under study. To permit comparison of death rates from year to year, a standard population is used. Beginning with the 1999 data year, NCHS adopted the year 2000 projected population of the United States as the standard population. This new standard replaces the 1940 standard population that was used by NCHS for over 50 years. The new population standard affects the level of mortality and to some extent trends and group comparisons. Of particular note are the effects on race comparison of mortality. For detailed discussion, see: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.Beginning with publications of the year 2003 data, the traditional standard million population along with corresponding standard weights to six decimal places were replaced by the projected year 2000 population age distribution (see 2000 Standard Population below). The effect of the change is negligible and does not significantly affect comparability with age-adjusted rates calculated using the previous method.

Publication Date: Accessed 21 May 2022, last reviewed 2 March 2022

Publication Site: CDC WONDER