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**Age Adjustment    of National Center for Health Statistics Data    Using the 2000 Projected U.S. Population    with Emphasis on Survey Data SystemsCharlotte Schoenborn,    Division of Health Interview Statistics    Richard Klein and Virginia Freid,    Office of Analysis, Epidemiology, and Health Promotion**

[NOTE: This paper was    prepared for NCHS internal use. However, the paper is being made available to outside data    users who may wish to construct comparable age-adjusted rates using the same standard    population. The paper will be adapted and published in the *Healthy People Statistical    Note* series later this year.]    

**Introduction**    Age adjustment, using the direct method, is the application of age-specific rates to a    standard age distribution in order to eliminate differences in observed crude rates in    populations of interest that result from differences in the populations' age    distributions.  This adjustment is usually done when comparing two or more    populations at one point in time or one population at two or more points in time. Age    adjustment is particularly helpful when populations being compared have very different age    structures. The classic literature on age adjusting,**1**    as well as more recent National Center for Health Statistics (NCHS) publications,**2-5** has focused on adjusting vital events (e.g.,    deaths, births) and provides comprehensive discussions of age adjustment techniques,    however the technique can be applied to any population-based events.

The purpose of this paper    is to establish several sets of age-adjustment weights, based on the year 2000 projected    U.S. population, that can be applied to analysis of NCHS survey data. This work builds on    the foundation set by the NCHS Division of Vital Statistics that established the year 2000    projected U.S. population as the Standard population for age adjusting mortality    statistics. The age groupings provided in this paper are based on the most common    groupings found in existing NCHS publications. Age adjustment weights, as well as examples    of SUDAAN code appropriate for calculating age-adjusted rates for complex sample surveys,    are being made available to NCHS staff and others. They are not intended as rules for age    adjustment, but rather as guidelines in order to promote and facilitate consistency in age    adjustment procedures among users of NCHS data.

**Historical Perspective**Several years ago NCHS    coordinated an effort that resulted in an agreement among certain Federal and State    agencies to age adjust mortality data using the year 2000 projected U.S. population.**3,4** In August 1998, the Secretary of the    Department of Health and Human Services (DHHS) issued a policy statement directing all    DHHS agencies to use the year 2000 projected U.S. population (hereafter referred to as the    2000 Standard Population) for age adjusting mortality rates beginning no later than data    year 1999. This agreement resolved long-standing problems associated with the use of    different standard populations for age adjusting death rates (1940, 1970, 1980, etc.) by    various agencies publishing mortality statistics, and the resultant difficulties    experienced by data users in comparing these rates.

At the time of the    decision to age adjust mortality data using the 2000 Standard Population, no decision was    made for other NCHS data systems (e.g., National Health Interview Survey, National Health    and Nutrition Examination Survey, and the various health care surveys such as the National    Hospital Discharge Survey and the National Nursing Home Survey). These data have continued    to be age adjusted using a variety of standard populations (1970 civilian,    noninstitutionalized population; 1980 resident population; 1990 civilian,    noninstitutionalized population, etc.). The various standard populations that have been    used in Health, United States (through publication year 1999) can be found in Appendix II    of *Health, United States.***7** In order to reduce    incomparability resulting from the use of a variety of standard populations and to    increase applicability of age-adjusted rates, some major DHHS reports will now publish    survey statistics that have been age adjusted using the 2000 Standard Population. Notable    among these publications are *Health, United States, 2000* and *Healthy People    2010 Objectives for Improving Health*.

**Calculation of Age    Adjustment Weights: Background, Process, and Procedures**The process of deriving    age adjustment weights for NCHS survey data began by    identifying age groupings typically utilized at NCHS for age adjustment and attempting to    ensure that all of the various groupings could be constructed from a single "NCHS    Master List" **(Table 1)**.

    The source of the population figures in the NCHS Master List was the official Bureau of    the Census projections for the year 2000 published in Current Population Reports series    P-25 no.1130, table 2.**8** This is the same    publication from which the standard population was derived for mortality data. The census    projections include breakdowns by sex, age, and race. However, only the totals for both    sexes and all races were used to develop the NCHS Master List. The complete **2000 Standard    Population projections** are available from the Bureau of the Census      web site.

    The NCHS Master List, consisting of 24 age groups, was developed with the goal of reducing    discrepancies that can occur when extracting and combining numbers from the published    Census projections. The published Census projections for the 2000 population--both for    single years of age and for 5-year age groups--are rounded to thousands.  However,    population sizes for the 5-year age groups were calculated by summing the unrounded counts    for single years (unpublished) and then rounding that sum. The result is that the    published sums of the single years of age do not necessarily add to the published 5-year    totals. As a consequence, age-adjustment weights based on the published Census projections    may be slightly different, depending on whether single-year or 5-year age groups are    combined to produce the weights. By defining the NCHS Master List as the starting point    for deriving age-specific population estimates and age-adjustment weights, such    discrepancies are minimized.

**Table    2** shows 15 age distributions that cover many of the current analytic needs of    existing standard NCHS reports. These distributions are displayed for the convenience of    data users. The actual choice of age groupings to be used for age adjusting is an analytic    decision and is not limited to the age distributions shown in **table    2**.

Analysts should remember    that the number and the ranges of age groups used for age adjusting may have a small    impact on the level of the age-adjusted rate. In choosing the number of age groups for a    specific analysis, two competing interests need to be addressed. First, the more age    groups utilized, the tighter the control of the effect of the variation in the age    distribution among groups or time periods being compared. On the other hand, when too many    age groups are used and the data become sparse for certain age groups, the resultant    age-specific rates used in the computation of the age-adjusted rate are less stable. It    should also be noted that the greater the difference between the age distribution of the    standard population used to age adjust (in this case, the 2000 Standard Population) and    the age distribution of the study population, the greater the difference between the crude    rate and the age-adjusted rate for the study population. When the age distribution of the    standard population and the age distribution of the study population are exactly the same,    the crude rate is equal to the age-adjusted rate.

When creating the NCHS    Master List, the age-specific population counts were made as consistent as possible with    the published Census projections. In some cases, minor adjustments to the age-adjustment    weights were necessary in order for the weights to sum to one. With the exception of Age    Distribution #1 (described below), age-specific counts for the age distributions shown in    **table 2** were derived directly from the NCHS    Master List (**table 1**). The totals for each    distribution were obtained by summing the component age-specific counts. Age-adjustment    weights for each of the age distributions were obtained by dividing the age-specific    counts by the total count over all ages in that age distribution and making any necessary    adjustments (where the impact would be the least) so that they summed to one. Note that,    regardless of whether the age distribution covers all ages or a subset of all ages (e.g.,    adults, children, and elderly), the weights are re-normalized to always sum to one.

Age Distribution #1 shown    in **table 2** consists of age groups and    age-adjustment weights previously published by the NCHS Division of Vital Statistics and    used for age adjusting mortality statistics.**4**    Note that an erratum sheet to this publication exists that shows the weight for the 75-84    year-old group as 0.044842, instead of 0.044841 as initially printed. This minor    difference is an example of the adjustments that must be made to the actual weight in    order for the weights to sum to 1. The actual calculated weight was 0.044841498, which    when rounded to six decimal places, was 0.044841. However, using this weight, the sum of    all of the weights fell short of 1. By making a very minor adjustment and increasing the    weight to 0.044842, the age-adjustment weights summed to 1. A similar procedure was used    to obtain the weights for three of the age distributions (Age Distributions #2, #6, and    #10) presented in this paper. For Age Distribution #2 the weight for 40-49 years was    changed from 0.153969 to 0.153968; for Age Distribution #6 the weight for 50-59 years was    changed from 0.134220 to 0.134221; and for Age Distribution #10 the weight for 65 years    and over was changed from 0.293705 to 0.293704. In all cases, the age group selected for    modification was the one that, when changed, would have the least impact on the weight    calculation. No modification was required for the other age distributions.

These 15 age distributions    clearly cannot cover all possible situations that would require age adjustment. Analysts    desiring age breakouts not specified here can use the NCHS Master List to generate other    age distributions and their associated age-adjustment weights. In some cases the NCHS    Master List may not contain all of the age groups desired. Additional detail, including    age-sex-specific counts for use in age-sex standardization, is available from the original    census population projections.**8** When    extracting additional subpopulations from the census projections, the new standard    population counts should be as consistent as possible with the NCHS Master List, and the    weights of the newly constructed age distributions must sum to one.

**Age-Adjusted vs. Crude    Rates: Definitions and Examples**Suppose that frequencies    of the occurrence of a characteristic (e.g., being a current cigarette smoker) have been    determined in each of several age groups for members of a study population (e.g., 1997    U.S. population, ages 18 and over). These are **age-specific frequencies** of    the characteristic (cigarette smoker). The **age-specific rates** of the    characteristic are the ratios of the **age-specific frequencies** to the    respective age-specific sizes of the study population.

The **crude rate**    of the characteristic is the total number of occurrences of the characteristic (cigarette    smoker) across all age groups divided by the total study population (all persons aged 18    and over).

Suppose that population    sizes for the same age groups have been obtained for a selected standard population. Then    the **age-adjusted rate** of the characteristic is simply a weighted    average of the **age-specific rates**. The **weight** for a    given age group is the size of the corresponding age group in the Standard population    divided by the total population. The weights thus add to one. The **age-adjusted    rate** has the advantage (over the **crude rate**) of being controlled    for age when compared to age-adjusted rates for other study populations that have    different age structures. (**Age-adjusted rates** being compared must all be    based on the same Standard population.)

To illustrate, suppose one    wants to examine smoking rates by level of education.  Because the age distributions    of various education groups differ, it is desirable to age adjust the rates in order to    ensure that any education differences that are observed are not confounded by differences    in the age structure of the various education groups. In this example, each education    group (e.g., less than high school, high school graduate, etc.) is considered to be a    separate study population, and the smoking rate for each is age adjusted. Examples of the    calculation of age-adjusted smoking prevalence rates by level of education are shown in    tables A and B. **Table A******(****View/download PDF****6 KB) illustrates these calculations using    age-specific population frequencies from the 2000 Standard Population; **table B**(****View/download    PDF****6 KB)  illustrates    these calculations using age-adjustment **weights**. The age-adjustment **weights**    in table B were rounded to 6 decimal places but could have been rounded to any number,    with the possibility of slightly different results.
**Age Adjustment using SUDAAN    software**When analyzing data from    complex sample surveys, the sample design must be taken into account. SUDAAN is the    software package used most often by NCHS analysts in order to calculate variance estimates    for NCHS survey data. This section provides SUDAAN code that can be used to age adjust    survey data.

Either age-specific    frequencies or the age-adjustment weights shown in **table 2**    (or derived from the NCHS Master List) may be used to produce age-adjusted rates using    SUDAAN's PROC DESCRIPT procedure. All of the calculations shown in tables A and B can be    accomplished with just two lines of SUDAAN code added to the existing PROC DESCRIPT code.

Figure 1 shows PROC    DESCRIPT code using Standard Population Age Distribution # 8 from **table    2**. The example variable (AGEGRP8) is a recode based on all ages of National    Heath Interview Survey sample adults and consists of four age categories: 18-24 years,    25-44 years, 45-64 years, and 65 years and over. The numbers shown after the word STDWGT    represent the number of persons in each of these age categories in the projected 2000    Standard Population, rounded to 1,000's. The age distribution of AGEGRP8 and the age    distribution of the Standard population (Age Distribution #8) must match. SUDAAN uses    these age-specific population frequencies to calculate the age-adjustment weights and then    produces an age-adjusted rate for the total adult population, ages 18 and over.

**Figure 1: SUDAAN    code to produce age-adjusted rates, using Standard population estimates (1,000's).**

PROC DESCRIPT;     NEST     WEIGHT     VAR     CATLEVEL **STDVAR AGEGRP8 ;****STDWGT 26258 81892 60991 34710 ;**    SUBGROUP     LEVELS     TABLES 

    Figure 2 shows PROC DESCRIPT code, using the same variable as above (AGEGRP8) and the same    4 age categories: 18-24 years, 25-44 years, 45-64 years, and 65 years and over.     However, in place of the age-specific Standard population frequencies shown in the STDVAR    statement in figure 1, this version uses the age-adjustment weights. These age-adjustment    weights represent the proportion each group represents of the total Standard population    ages 18 and over (rounded to six decimals) and correspond to the categories of the AGEGRP8    variable on the data file (i.e., ages 18-24 years (0.128810); ages 25-44 years (0.401725);    ages 45-64 years (0.299194); and ages 65 years and over (0.170271)). **Figure 2: SUDAAN code to produce age-adjusted rates, using age-adjustment weights**

    PROC DESCRIPT;     NEST     WEIGHT     VAR     CATLEVEL **STDVAR AGEGRP8 ;     STDWGT .128810 .401725 .299194 .170271 ;**    SUBGROUP     LEVELS     TABLES     The standard population age groupings must match those of the study population. For some    survey data, adjustment to the total population (i.e., all ages) is appropriate. In other    cases, such as the example used here, subsets of the all-ages population are the    appropriate standard (e.g., persons 18 years and over for the NHIS Sample Adult    questionnaire). It should be kept in mind that in some, selected cases, the 2000 Standard    Population may not be the most appropriate standard, such as in studies of nursing home    populations. However, to the extent that analyses based on NCHS data systems employ the    same standard population for age adjustment, it will be easier to compare and interpret    results across data systems and across data years. Using age adjustment standards other    than the 2000 Standard Population should be appropriately justified.    

    This document was a collaborative effort of the Division of Health Interview Statistics    and the Office of Analysis, Epidemiology, and Health Promotion and is being made available    to all CDC staff via the NCHS Intranet at: http://inside.nchs.cdc.gov/oaehp/ageadj.htm.    

**References**1. Shryock HS, Siegel JS, et al. The methods and materials of demography.      Academic Press. New York. 1976.2. Feinleib M, Zarate AO (eds). Reconsidering age adjustment procedures:    Workshop proceedings. Vital Health Stat 4(29). 1992.3. Anderson RN, Rosenberg HM. Report of the second workshop on age    adjustment.  National Center for Health Statistics. Vital Health Stat 4(30). 19984. 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.****View/download PDF1.1 MB5. Curtin LR, Klein RJ. Direct standardization (age-adjusted death rates).    Statistical Notes No 6 (Revised). March 1995.6. Shalala DE. HHS Policy for changing the population standard for age    adjusting death rates. Memorandum from the Secretary. August 26, 1998.7. National Center for Health Statistics. **Health, United    States, 1999, With Health and Aging Chartbook**. Hyattsville, Maryland. 1999.8. Day, JC. Population projections of the United States by age, sex, race,    and Hispanic origin: 1995 to 2050, U.S. Bureau of the Census, **Current Population Reports**,    P25-1130, U.S. Government Printing Office, Washington, DC. 1996. 
    Date last modified: July 11, 2000 

 
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This page last reviewed August 30, 2000

**U.S.    DEPARTMENT OF HEALTH AND HUMAN SERVICES****Centers    for Disease Control and Prevention**    National Center for Health Statistics    Division of Data Services     Hyattsville, MD    20782-2003    (301) 458-4636
 

 

 

 

 

 

 

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