Number 22 September 2001

Summary Measures of Population Health:

Addressing the First Goal of Healthy People

2010, Improving Health Expectancy

Diane K. Wagener, Ph.D.; Michael T. Molla, Ph.D.; Eileen M. Crimmins, Ph.D.; Elsie Pamuk, Ph.D.;

and Jennifer H. Madans, Ph.D.

Abstract

This report is one of several Healthy People Statistical Notes

that evaluate methodological issues pertaining to summary

measures—statistics that combine mortality and morbidity

data to represent overall population health in a single number.

This report evaluates the consequences of changes in the components of health expectancy measures (i.e., mortality or morbidity) on the overall measure. Any activity limitation is used

as a morbidity measure. Simulations are used to evaluate the

impacts of reducing 1995 age-specific mortality or activity

limitation rates by 5, 10, 25, and 50 percent at all ages. Then

it is limited to ages under 25 years, 25–64 years, and over 64

years. The impact of completely eliminating mortality or

activity limitation for the younger age groups is also examined. In general, reducing morbidity rates results in greater

changes than the same percent reduction in death rates. The

same proportional reduction in age-specific rates for either

mortality or morbidity has a greater impact if it occurs at

older ages. Reducing mortality results in a greater change in

life expectancy than in health expectancy and a decline in the

proportion of life lived in healthy states. Reducing morbidity

increases both health expectancy and the proportion of life

lived in healthy states. Simultaneous reductions in mortality

and morbidity have additive effects on health expectancy.

Keywords: summary measures c health expectancy c

disability-free life expectancy c life expectancy

Introduction

The increase in life expectancy during the 20th century

has been a remarkable achievement, rising from 47 years in

1900 to almost 76 years in 1995 (1), an increase of more

than 60 percent. One of the overarching goals of Healthy

People 2010 (2) is to increase the quality and years of

healthy life. This goal targets not only a continued increase

in life expectancy (dependent on reduced mortality), but also

an improvement in the percent of life lived with good health

(dependent on reduced morbidity). Therefore, summary

measures of population health are being evaluated as

statistics that could be used to monitor success in addressing

this goal. Summary measures of population health are

defined by the Institute of Medicine as measures ‘‘that

combine both mortality and morbidity data to represent

overall population health in a single number (e.g.,

health-adjusted life years or health-adjusted life

expectancy)’’ (3). More specifically, summary measures of

population health (hereafter referred to as ‘‘summary

measures’’) combine age-specific morbidity (health) and

mortality rates to derive a single measure of population

health. These measures have been proposed as the most

comprehensive measures for evaluating overall trends and

differences in population health (4).

Public health programs and policies are designed to

reduce mortality and morbidity in the population. This report

evaluates the effects of change in these two components of

summary measures. No specific public health program or

policy is evaluated; hypothesized, simple percent changes in

age-specific mortality or morbidity rates at all ages and for
specific age groups are examined. These analyses provide a 

sense of the magnitude of changes in summary measures of 

population health that might be expected for public health 

programs. Barendregt (5) evaluated disease-specific changes 

and the consequences for disability-adjusted life expectancy 

for five diseases having major consequences for mortality 

and morbidity. The consequences of reducing mortality and 

morbidity broadly are examined. 

Many potential summary measures have been 

proposed (4). As part of Healthy People 2000, the Years of 

Healthy Life measure was developed to monitor a similar 

goal (6,7). This measure combines the age-specific mortality 

rates with a morbidity variable that is a weighted average of 

respondent-assessed health status and limitation of activity 

caused by chronic health conditions. Because the 

morbidity-related variable is multivariate, interpretation of 

sensitivity analyses is complicated. However, the analyses 

presented in this report focus on a simple health-adjusted life 

expectancy measure (hereafter referred to as health 

expectancy) that combines total mortality and a univariate 

measure of morbidity using the methods described by 

Sullivan (8). Details on the calculation of health expectancy 

measures have been published (9). Health expectancy 

statistics are presented for reports of any activity limitation 

due to impairments or health problems, largely chronic 

health conditions. Because activity limitation is used as an 

indicator of disability, these health expectancy statistics may 

also be referred to as disability-free life expectancy. 

Health expectancy is often interpreted as the number of 

years an average person is expected to live in good health. 

Because populations usually have less than good health, the 

number of expected healthy years is less than expected life 

years. Consequently, the difference is sometimes referred to 

as unhealthy years. All the unhealthy years are not 

necessarily lived at the end of life and could be distributed 

throughout a lifetime. Health expectancy, when viewed in 

the context of total life expectancy (i.e., as a proportion of 

total life expectancy), can also be interpreted as the 

proportion of life lived in a healthy state. On an individual 

level, an increase in the proportion of healthy life is an 

increase in the percent of an average year that is lived in a 

healthy state. On a population level, health expectancy as a 

proportion of life expectancy can be interpreted as the 

average proportion of the population that is healthy in a 

given year. An increase, therefore, would be an increase in 

the proportion of the population experiencing healthy life. 

It is important to note that as with life expectancy, 

health expectancy does not refer to the health experience 

over a lifetime of an actual population. Health expectancy is 

the number of years a hypothetical population would live in 

a given healthy state if it experienced throughout life the 

mortality and morbidity rates of the specific year being 

considered, in this 1995 report. 

In this report, different sets of assumptions about change 

in the components are systematically applied to alter real 

data to assess the resulting changes in health expectancy. 

Many models might be proposed for reducing mortality 

and/or morbidity rates. Simple models of equal proportional 

reductions in either mortality or morbidity for specific ages 

are used. Crimmins et al. (10) presented similar analyses to 

evaluate consequences on summary measures of health for 

adults aged 70 years and older. In this segment of the 

population, reducing mortality without reducing morbidity 

increases both the years and the proportion that is unhealthy 

life. Conversely, reducing morbidity without reducing 

mortality reduced both the years and the proportion of life 

that is unhealthy. In the following analyses, changes in 

age-specific mortality or age-specific activity limitation due 

to chronic health conditions are applied separately to four 

categories: all ages, children and young adults (less than age 

25 years), adults (ages 25–64 years), and older adults 

(65 years and older). These analyses are evaluated for their 

impact on summary measures of population health. 

Hypothetical reductions are applied to mortality rates for 

1995, to activity limitation rates for 1995, and jointly to the 

mortality and activity limitation rates. In each section, the 

impact on life expectancy and on years free of activity 

limitation (the health expectancy measure evaluated in this 

report), the relative impact of changing the size of the 

reduction, and the relative impact of reductions in the 

different age groups are discussed. These impacts are then 

compared to the observed changes in life expectancy and 

years free of activity limitation for the decade between 1985 

and 1995. 

Changes in Life and Health

Expectancy, 1985 to 1995

To gauge the size of changes in age-specific mortality or 

morbidity rates that are possible, changes that have occurred 

in the U.S. population were explored. As noted above, life 

expectancy has changed dramatically in the past century. 

Between 1900 and 1995, life expectancy at birth increased 

57 and 63 percent, respectively, for males and females (from 

46.3 to 72.5 years for males and from 48.3 to 78.9 for 

females (1)). Figures 1 and 2 compare age-specific total 

mortality rates used to calculate the expected years of life 

for 1900 and 1995, presented separately for males and 

females (see table I). Remarkable decreases over time are 

noted among children, particularly those under 5 years of 

age (about a 97-percent decrease for both sexes). Although 

changes are apparent for those aged 25–64 years, the 

magnitude of the death rates is relatively small. Among 

those aged 65 years or older, age-specific mortality 

decreased 35 to 45 percent among men and 40 to 55 percent 

among women between 1990 and 1995. 

Although substantial changes occurred early in the 

century, changes in mortality, and consequently life 

expectancy, continued in the recent past. Table 1 presents 

age-adjusted mortality for 1985 and 1995. During this 

decade, mortality decreased 8.2 percent, from 548.9 per 

100,000 population in 1985 to 503.9 in 1995 (1). For 

subpopulations, the change over the decade varied. 

Reductions in mortality were greatest among white males 

2
Under 

1 year 

1-4 

years 

5-14 

years 

15-24 

years 

25-34 

years 

35-44 

years 

45-54 

years 

55-65 

years 

65-74 

years 

75-84 

years 

85 years 

and over 

1900 1995 

Age 

0 

5,000 

10,000 

15,000 

20,000 

25,000 

30,000 

Deaths, per 100,000 population 

SOURCE: National Vital Statistics System, Centers for Disease Control and Prevention, National Center for Health Statistics. 

Figure 1. Death rates among males by age: United States, 1900 and 1995

0 

5,000 

10,000 

15,000 

20,000 

25,000 

30,000 

Deaths, per 100,000 population 

Under 

1 year 

1-4 

years 

5-14 

years 

15-24 

years 

25-34 

years 

35-44 

years 

45-54 

years 

55-65 

years 

65-74 

years 

75-84 

years 

85 years 

and over 

1900 1995 

Age 

SOURCE: National Vital Statistics System, Centers for Disease Control and Prevention, National Center for Health Statistics. 

Figure 2. Death rates among females by age: United States, 1900 and 1995

3
Table 1. Mortality rate and percent of people with any limitation of activity due to chronic health conditions, by race and sex:

United States, 1985 and 1995

Total mortality1 Any activity limitation1

Population 1985 1995 n2 Percent n3 1985 1995 n2 Percent n3

Deaths per 100,000 population Percent

Total........................................ 548.9 503.9 –45.0 –8.2 12.4 13.1 0.7 5.3

White

Males ....................................... 693.3 610.5 –82.8 –11.9 12.4 13.1 0.7 5.7

Females ..................................... 391.0 364.9 –26.1 –6.7 11.6 12.5 0.9 7.4

Black

Males ....................................... 1,053.4 1,016.7 –36.7 –3.5 15.1 17.0 1.9 13.2

Females ..................................... 594.8 571.0 –23.8 –4.0 15.2 16.1 0.9 5.5

1Mortality and frequency of activity limitation due to chronic health conditions are age-adjusted to 1940 standard population. 2n is the difference between 1985 and 1995: 1995 value - 1985 value. 3Percent n is the percent difference between 1985 and 1995: n /1985 value.

SOURCES: Centers for Disease Control and Prevention, National Center for Health Statistics. National Vital Statistics System (mortality); National Health Interview Survey (noninstitutionalized

population); National Nursing Home Survey (nursing home population).

and least among black males. Because mortality decreased 

between 1985 and 1995, life expectancy at birth for the total 

population increased 1.1 years to 75.8 years, an increase of 

1.5 percent (table 2). White males had the largest increase, 

and black males had the least increase. For white and black 

females, the increase in expected life was about 1 percent. 

Health change may be viewed as a process with 

multiple dimensions: diseases, conditions, and impairments; 

loss of function; disability; and death (11). Over time, 

change can occur in some or all of these dimensions. The 

incorporation of mortality in the calculation of health 

expectancy assures that changes in the death dimension are 

reflected in the measure. However, there are many choices 

of morbidity measures that could be used in summary 

measures. In this report, any activity limitation due to 

chronic health conditions are used. This is a measure of 

disability. Figure 3 presents age-specific rates for mortality 

and activity limitation in 1995 (table II). 

The proportion of the population with activity limitation 

due to chronic health conditions (hereafter referred to as 

activity limitation) increased between 1985 and 1995 

(table 1). For the total population, the age-adjusted 

proportion increased 5.3 percent. Large differences in 

activity limitation occur for subgroups of the population. In 

1995, the percent of black males reporting any activity 

limitation due to chronic health conditions was 1 1/3 times 

the rate for white females. Further, the increases were the 

largest for black males, both in terms of actual increase and 

percent increase. They were the smallest for white males. 

The increase among black males was almost 3 times that of 

white males. 

Using age-specific mortality and activity limitation, 

the health expectancy statistic (expected years free of 

activity limitation) can be calculated utilizing life table 

procedures (see appendix and Healthy People Statistical 

Notes no. 21 (9)). In 1995, expected years free of activity 

limitation for the total population was 62.2 years (table 2). 

This was a slight increase in expected years free of 

activity limitation (1.0 percent), although the total 

frequency of activity limitation increased (table 1). This 

Table 2. Life expectancy and number of expected years free of any activity limitation due to chronic health conditions, by race and sex:

United States, 1985 and 1995

Expected

years of life

Expected years free of

activity limitation

Population 1985 1995 n1 Percent n2 1985 1995 n1 Percent n2

Total........................................ 74.7 75.8 1.1 1.5 61.6 62.2 0.6 1.0

White

Males ....................................... 72.0 73.7 1.7 2.4 60.5 61.1 0.6 1.0

Females ..................................... 78.6 79.5 0.9 1.1 64.9 64.8 –0.1 –0.2

Black

Males ....................................... 65.0 65.4 0.4 0.6 53.3 52.6 –0.7 –1.3

Females ..................................... 73.2 73.9 0.7 1.0 57.2 57.4 0.2 0.3

1n is the difference (in years): 1995 value - 1985 value. 2Percent n is the percent difference between 1985 and 1995: n /1985 value.

SOURCES: Centers for Disease Control and Prevention, National Center for Health Statistics. National Vital Statistics System (mortality); National Health Interview Survey (noninstitutionalized

population); National Nursing Home Survey (nursing home population).

4
0 

100 

200 

300 

400 

500 

600 

700 Death rates Any activity limitation 

Rate per 1,000 population 

Under 

1 year 

1-4 

years 

5-14 

years 

15-24 

years 

25-34 

years 

35-44 

years 

45-54 

years 

55-65 

years 

65-74 

years 

75-84 

years 

85 years 

and over 

Age 

SOURCE: National Vital Statistics System, Centers for Disease Control and Prevention, National Center for Health Statistics. 

Figure 3. Death and reported activity limitation rates by age: United States, 1995

apparent contradiction can be attributed to a decrease in 

the other principal component of health expectancy, 

mortality. The change in expected years free of activity 

limitation differed between subgroups of the population. 

Expected years free of activity limitation increased among 

white males and black females, but decreased among the 

other race-gender groups. Among black males, expected 

years free of activity limitation decreased by 0.7 years. 

Effects of Changing Mortality

The following analyses determine how sensitive this 

summary measure is to reductions in mortality. To evaluate 

the effect of changing mortality on both life expectancy and 

expected years free of activity limitation, the effects of 

simulated changes were examined. Age-specific rates for 

designated age ranges were reduced proportionally, and the 

life and health expectancies were recalculated in a series of 

models. Reductions of 5, 10, 25, and 50 percent were 

applied to the observed 1995 5-year mortality rates (table II) 

for four sets of age groups—all ages, less than 25 years, 

25–64 years, and 65 years and older. In another model, 

complete elimination of mortality (i.e., a 100-percent 

reduction) was assessed for children and young adults and 

for adults 25–64 years old. For these analyses, morbidity 

rates—the other principle component of health 

expectancy—was not altered. The results of these analyses, 

using activity limitation as the measure of morbidity, are 

shown in table 3. 

In 1995, 35.6 percent of the population was less than 

25 years of age, 51.6 percent were ages 25–64 years, and the 

remaining 12.8 percent were aged 65 years or older (12). 

These age groups were chosen because the functions 

required for daily activities differ substantially among these 

age groups (e.g., school, work, or recreational activities). 

Another reason for choosing these age groups is that the 

most common adverse health conditions differ—as evidenced 

by the leading causes of death (1) (e.g., congenital anomalies 

and unintentional injuries among the young and malignant 

neoplasms and diseases of the heart among the middle and 

older age groups) and most frequent first-listed diagnosis on 

hospital discharge records (1) (e.g., pneumonia and injuries 

or poisonings among the young, injuries and diseases of the 

heart among the middle ages, and diseases of the heart, 

cerebrovascular diseases, and pneumonia among the older 

population). Consequently, the health experience for these 

three age groups differs and the evaluation of ‘‘quality 

health’’ also differs. 

Life Expectancy Versus Health Expectancy

Reductions in mortality have a larger effect (i.e., a 

greater increase) on life expectancy than on expected years 

free of activity limitation (table 3). For example, focusing on 

the models in which mortality is reduced at all ages, a 

5-percent reduction increases life expectancy by 0.6 years, 

but expected years free of activity limitation by only 0.3 

years. When mortality is reduced 50 percent, the resulting 

5
Table 3. Life expectancy and number of expected years free of any activity limitation due to chronic health conditions by several models

of reduced mortality without reduced activity limitation

Expected Expected years free of

years of life activity limitation

Percent

Model1 Years n2 Percent n3 Years of LE4 n2 Percent n3

No reduction in age-specific mortality

Observed5 ......................................... 75.8 . . . . . . 62.2 82.1 . . . . . .

5-percent reduction in age-specific mortality

All ages .......................................... 76.4 0.6 0.8 62.5 81.8 0.3 0.5

Under 25 years ..................................... 75.9 0.1 0.1 62.2 81.9 0.0 0.0

25–64 years ....................................... 76.1 0.3 0.4 62.3 81.8 0.1 0.2

Over 64 years ...................................... 76.1 0.3 0.4 62.3 81.8 0.1 0.2

10-percent reduction in age-specific mortality

All ages .......................................... 77.0 1.2 1.6 62.8 81.6 0.6 1.0

Under 25 years ..................................... 76.0 0.2 0.3 62.3 82.0 0.1 0.2

25–64 years ....................................... 76.3 0.5 0.7 62.5 81.9 0.3 0.5

Over 64 years ...................................... 76.3 0.5 0.7 62.4 81.8 0.2 0.3

25-percent reduction in age-specific mortality

All ages .......................................... 78.8 3.0 4.0 63.9 81.1 1.7 2.7

Under 25 years ..................................... 76.2 0.4 0.5 62.4 81.9 0.2 0.3

25–64 years ....................................... 77.1 1.3 1.7 63.0 81.7 0.8 1.3

Over 64 years ...................................... 77.2 1.4 1.8 62.8 81.3 0.6 1.0

50-percent reduction in age-specific mortality

All ages .......................................... 82.2 6.4 8.4 65.9 80.2 3.7 5.9

Under 25 years ..................................... 76.5 0.7 0.9 62.7 82.0 0.5 0.8

25–64 years ....................................... 78.3 2.5 3.3 63.8 81.5 1.6 2.6

Over 64 years ...................................... 78.7 2.9 3.8 63.6 80.8 1.4 2.3

Complete elimination of age-specific mortality

Under 25 years ..................................... 77.2 1.4 1.8 63.3 82.0 1.1 1.8

25–64 years ....................................... 81.0 5.2 6.9 65.5 80.9 3.3 5.3

. . . Category not applicable.

0.0 Quantity more than zero but less than 0.05. 1The models are proportional reductions of a specified percent for age-specific death rates in four age-group categories: all ages, ages under 25 years, ages 25 to 64 years, and ages 65 years

and older. Complete elimination (100 percent reduction) is assumed for the younger age groups. 2n is the difference from observed (in years): Model estimate - Observed years. 3Percent n is the percent difference from observed: n /Observed years. 4LE is expected years of life. 5Observed values are from 1995.

changes are more disparate, 6.4 and 3.7 years, respectively. 

The explanation for this finding is that, as part of the health 

expectancy calculation, each year of life is multiplied by the 

percent of the population free of activity limitation (i.e., each 

year of life is discounted by the percent of the population 

living in unhealthy states) (see the appendix). Therefore, a 

decrease in mortality results in an increase in total years for 

life expectancy, but only an increase in the healthy portion 

of those years for health expectancy. For example, if 

reducing mortality results in an increase of 10 years of life, 

x of those years may be unhealthy and 10 - x healthy. 

Consequently, the effect of reducing mortality will be greater 

on life expectancy, which is calculated using total years, 

rather than on years spent free of activity limitation. The 

latter is calculated using only healthy years. 

Impact of Changing Size of Reduction

As the reduction of death rates gets greater (e.g., from a 

5-percent to a 50-percent decrease), the increases in life and 

limitation-free expectancy become proportionally larger. 

Consider decreases in death rates for adults ages 25–64 

years in table 3. As the age-specific mortality is reduced by 

5, 50, and 100 percent, the life expectancy increases 0.4, 3.3, 

and 6.9 percent. Expected years free of activity limitation 

improved by slightly smaller proportions (0.2, 2.6, and 

5.3 percent, respectively). 

Years free of activity limitation as a proportion of total 

life expectancy (i.e., the percent of life lived free of activity 

limitation) decreases with larger reductions in mortality. 

Overall in 1995, expected years free of activity limitation as 

a proportion of total life expectancy was 82.1 percent 

(table 3). Consider mortality reductions for all ages. With a 

5-percent decline in mortality, the proportion becomes 

81.8 percent; with a 25-percent decline, it becomes 

81.1 percent; and with a 50-percent decline, it becomes 

80.2 percent. This decrease in the proportion of life free of 

activity limitation, in the face of reducing mortality, occurs 

because much of the gain in mortality occurs in the older 

6
Table 4. Number of expected years free of any activity limitation due to chronic health conditions by several models of reduced activity

limitation without reduced mortality

Difference Percent

from change from

Percent observed observed

Model1 Years of LE2 value value

No reduction in age-specific morbidity

Observed3 .............................................. 62.2 82.1 . . . . . .

5 percent reduction in age-specific morbidity

All ages ............................................... 62.8 82.8 0.6 1.0

Under 25 years .......................................... 62.2 82.1 0.0 0.0

25–64 years ............................................ 62.5 82.4 0.3 0.5

Over 64 years ........................................... 62.4 82.3 0.2 0.3

10 percent reduction in age-specific morbidity

All ages ............................................... 63.5 83.8 1.3 2.1

Under 25 years .......................................... 62.3 82.2 0.1 0.2

25–64 years ............................................ 62.8 82.8 0.6 1.0

Over 64 years ........................................... 62.7 82.7 0.5 0.8

25 percent reduction in age-specific morbidity

All ages ............................................... 65.6 86.5 3.4 5.5

Under 25 years .......................................... 62.5 82.5 0.3 0.5

25–64 years ............................................ 63.7 84.0 1.5 2.4

Over 64 years ........................................... 63.6 83.9 1.4 2.3

50 percent reduction in age-specific morbidity

All ages ............................................... 69.0 91.0 6.8 10.9

Under 25 years .......................................... 62.9 83.0 0.7 1.1

25–64 years ............................................ 65.3 86.1 3.1 5.0

Over 64 years ........................................... 65.1 85.9 2.9 4.2

Complete elimination of age-specific morbidity

All ages ............................................... 75.8 100.0 13.6 21.9

Under 25 years .......................................... 63.7 84.0 1.5 2.4

25–64 years ............................................ 68.5 90.4 6.3 10.1

Over 64 years ........................................... 67.9 89.6 5.7 9.2

. . . Category not applicable.

0.0 Quantity more than zero but less than 0.05. 1The models are proportional reductions of a specified percent for age-specific death rates in four age-group categories: all ages, ages under 25 years, ages 25 to 64 years, and ages 65 years

and older. Complete elimination (100 percent reduction) is assumed for the younger age groups. 2LE is expected life years. In 1995 the life expectancy was 75.8 years. Changing age-specific morbidity has no effect on expected life years. 3Observed values are from 1995.

age groups that have greater frequency of activity limitation. 

The correlation between age-specific rates of reported 

activity limitation and mortality in 1995 was 0.95 (Spearman 

rank-order correlation coefficients). The years gained, 

therefore, are more likely to be spent in poor health than the 

average life year of the population. For example, with a 

50-percent decline in death rates at all ages, the expected 

number of life years gained is 6.4 years, but the expected 

number of years gained free of activity limitation is only 3.7 

years. That is, only 57 percent of the years gained by 

reducing mortality (3.7 years/6.4 years) would be lived free 

of activity limitation if no change in activity limitation rates 

occurred. This proportion is less than the proportion for the 

overall population in 1995 (i.e., 82.1 percent). So the years 

gained are more likely to be lived in poor health compared 

with average life years. 

Mortality Reductions for Limited Age Ranges

The effect of reducing death rates is substantially less 

when the reduction in mortality occurs in a limited age 

range. A 50-percent decrease in mortality among children 

and young adults (under 25 years old) increases health 

expectancy by only 0.5 years free of activity limitation, 

about one-eighth the change if death rates for all ages are 

affected (3.7 more years free of activity limitation). The 

pattern is similar for resulting changes in life expectancy. 

Further, when the death rates among older age groups only 

are reduced, the increase in either life or limitation-free 

expectancy is about 3 times larger than when death rates 

among younger ages only are affected. The greater expected 

years increase occurs because mortality rates are generally 

higher for the older ages. Consequently, the same 

proportional reduction in mortality among children (for 

example, a 50-percent reduction) is a smaller absolute 

decrease in mortality because the mortality rates among 

children are much smaller than among either of the older 

age groups. The calculation of expected life years is, 

effectively, a weighted average of the death rates (and, for 

health expectancy, a weighted average of mortality and 

7
0 

1 

1 1 

2 

3 

4 

5 

6 

7 

8 

0 0 40 60 

Percent reduction 

Number of years

Mortality Activity limitation 

The number of years is the difference between two calculations of expected years 

free of activity limitation. First, assume a reduction in mortality or activity limitation 

and calculate expected years. Expected years are calculated from birth. 

2

Figure 4. Change in expected years free of activity limitation by

proportional reductions in either mortality or activity limitation

morbidity). Therefore, a smaller change in death rates would 

have a smaller change in the calculated expected years. 

When reductions are limited to certain age groups, the 

increase in expected years free of activity limitation is 

greatest when the reduced death rates occur among adults 

aged 25–64 years although the increase in expected life 

years is greatest when mortality reduction occurs among the 

older population. In the older population, the rates of 

morbidity are higher than either of the two younger 

segments of the population. Therefore, although a reduction 

in death rates among the older population results in a greater 

number of total life years gained, those years are more likely 

to be spent in poor health. Therefore, the total number of 

quality life years gained is, in fact, less than those gained 

when death rate reductions occur in the 25–64 year-old 

population. 

Observed Changes Between 1985 and 1995

The changes shown in table 3 are not strictly 

comparable to those in table 2. The models in table 3 assume 

1995 mortality and activity limitation rates and project future 

changes, whereas the data in table 2 reflect past changes. 

However, table 3 can be used to evaluate explanations for 

the health expectancy changes shown in table 2. 

The decade between 1985 and 1995 had an 8-percent 

reduction in mortality (table 1). Assuming a comparable, 

modest reduction model of 10 percent for all ages, the years 

free of activity limitation would be expected to increase 0.6 

years (table 3). This is the same as the observed increase of 

0.6 years free of activity limitation between 1985 and 1995 

(table 2). Therefore, the reductions in mortality alone could 

8

account for the magnitude of change in years free of activity 

limitation observed in this decade. 

Effect of Changing Morbidity

The two principal components of health expectancy are 

mortality and morbidity. Models of reduction similar to the 

mortality models in the previous section can be applied to 

age-specific activity limitation rates. The sensitivity of 

limitation-free expectancy to reduced activity limitation is 

evaluated below. 

Figure 3 shows the relative magnitudes of the two 

variables (mortality and frequency of reported activity 

limitation due to chronic health conditions) by age and sex. 

The rates of activity limitation are higher than that of 

mortality at all ages (rates given in table II). Even at the 

oldest ages, the rate of activity limitation was 3.5 times the 

mortality rate. 

To evaluate the sensitivity of limitation-free expectancy 

to reduction in age-specific activity limitation, reductions of 

5, 10, 25, 50, and 100 percent were applied to the observed 

1995 frequencies of any activity limitation for four sets of 

age groups—all ages, less than 25 years, 25–64 years, and 

65 years and older. The results of these analyses are shown 

in table 4. 

Life Expectancy Versus Health Expectancy

For these analyses, mortality was not altered. Because 

life expectancy calculation depends only on mortality, under 

these models with change in morbidity only, life expectancy 

does not change from the 1995 level of 75.8 years. Changes 

in morbidity affect the length of life in a given quality, but 

not life expectancy. Consequently, expected years free of 

activity limitation as a proportion of expected years of life 

will always increase as activity limitation is reduced (i.e., 

expected quality years will increase but expected life years 

remains unchanged). 

Impact of Changing Reduction Size

The effects on the number of expected years free of 

activity limitation when age-specific activity limitation is 

reduced (table 4) are similar to those noted for decreases in 

mortality (table 3). Larger reductions in activity limitation 

result in larger increases in limitation-free expectancy. As 

was the case when mortality was reduced, the relative 

impact on limitation-free expectancy is roughly proportional 

to the percent decrease for activity limitation. For example, 

when all ages are affected, a 5-percent decrease yields a 

change in limitation-free expectancy (0.6 years, table 4) that 

is one-tenth the change produced by a 50-percent decrease 

(6.8 years free of activity limitation). 

However, when compared to declines in mortality, the 

impact of reducing age-specific activity limitation on 

limitation-free expectancy as a percent of life expectancy is 

different. In 1995, 82.1 percent of the expected life years 

were lived free of activity limitation. When activity 

limitation is reduced by 5 percent for all ages, the proportion 
becomes 82.8 percent; when it is reduced by 50 percent, the 

proportion is 91.0 percent; and complete elimination of 

activity limitation yields a health expectancy equal to life 

expectancy of 100 percent. Consequently, the maximum 

increase in expected years free of activity limitation based 

on 1995 data, assuming no change in mortality, is 

22 percent, or 14 years. In summary, as expected, when 

activity limitation is reduced, the proportion of life lived free 

of activity limitation improves, whereas, when mortality is 

reduced, the proportion of life lived free of activity 

limitation decreases. 

The magnitude of the changes in limitation-free 

expectancy for similar models using age-specific activity 

limitation are greater than changes effected using mortality. 

For example, a 50-percent decline in mortality at all ages 

produces an increase in expected years free of activity 

limitation of 3.7 years (table 3), but a 50-percent reduction in 

activity limitation produces an increase of 6.8 years (table 4). 

The impact on the expectancy is greater when activity 

limitation is altered because the magnitude of the rates of 

activity limitation is much higher than mortality at all ages. 

Morbidity Reduction for Limited Age Ranges

Reducing activity limitation among the younger, middle, 

and older age groups of the population will result in changes 

in limitation-free expectancy similar to the changes resulting 

from reductions in mortality in these same age groups. That 

is, reducing activity limitation among children and young 

adults results in smaller changes in years free of activity 

limitation than either of the older age groups because the 

percent of the younger population with activity limitation is 

smaller than in older age groups. However, improvements in 

health expectancy are greatest when activity limitation is 

reduced among adults 25–64 years of age, compared with 

younger and older populations. Older populations have larger 

age-specific activity limitation rates so a proportional 

reduction would produce a larger change in age-specific 

activity limitation. However, the older age group is a smaller 

subgroup of the population. Thus the net effect on change in 

overall expected years free of activity limitation is smaller. 

The impact on years free of activity limitation is, therefore, 

a function of both the magnitude of the reduction and the 

proportion of the total population who benefit from the 

reduced activity limitation. 

Observed Changes Between 1985 and 1995

The frequency of any activity limitation actually 

increased between 1985 and 1995 (5.3 percent, table 1). 

Expected years free of activity limitation, nevertheless, did 

increase (0.6 year, table 2) due to an 8-percent decline in 

mortality. If, instead of an increase, activity limitation had 

either not changed or had been reduced by a comparable 

5 percent at all ages, larger increases in expected years free 

of activity limitation could have been achieved. 

Combined Changes in Mortality and

Morbidity

Figure 4 displays some of the findings reported above, 

that is, the gain in years free of activity limitation as a 

function of either reductions for all ages in mortality 

(table 3) or activity limitation (table 4). However, changes in 

mortality or morbidity rarely occur separately. Indeed, the 

Spearman rank-order correlation of age-specific rates of 

mortality and activity limitation for the 5-year age groups is 

almost 1.0 for 1995 (see above). Therefore, a change in one 

component is often associated with a change in the other. 

The joint impact of declines in both mortality and activity 

limitation are presented in table 5. As expected, the effect on 

life expectancy of reducing mortality only is the same as the 

effect of reducing both mortality and activity limitation 

(i.e., morbidity does not have an additional effect on life 

expectancy). However, reducing activity limitation in 

conjunction with a reduction in mortality has an additive 

effect on the years free of activity limitation. The 

consequences of combined reductions more than double the 

changes resulting from reductions in mortality only because 

the consequences of separately reducing activity limitation 

has greater impact than changing mortality. On the other 

hand, if activity limitation is actually increased when 

mortality is reduced, the expected years free of activity 

limitation can actually decrease. Again, the impacts are 

additive. 

As noted, the effects of changes in mortality and 

morbidity on health expectancy are effectively additive, 

although the probabilities of surviving and probabilities of 

remaining healthy are sequentially multiplied in the life table 

procedures (see appendix). However, because the mortality 

rates are small for most age groups, these combined effects 

are essentially additive. That is: 

�1�μx) (1−mx)=1−μx −mx + μx mx ≈ 1− μx − mx , 

where μx is the mortality at age x and mx is the morbidity of 

the same age group. 

Discussion

Major components of summary measures of population 

health—the age-specific mortality and morbidity rates—are 

considered in this report. The relative impact of changing 

any component depends on the average age-specific rate, the 

distribution of rates, and the proportion of the population 

that is affected. In general, changing higher age-specific 

rates (e.g., morbidity as compared with mortality) or 

changing age-specific rates at older ages (which generally 

have higher age-specific rates) will have greater impact on 

the summary measure. Larger changes in any component 

will result in larger changes in the summary measure. In 

summary, the larger the proportion of the total population 

whose lives are improved, the larger the impact. These 

conclusions are independent of the hypothesized model. 

As noted by Crimmins et al. (10) for the elderly 

population, declining mortality increases life expectancy and 

9
Table 5. Life expectancy and number of expected years free of any activity limitation due to chronic health conditions by several models

of reduced mortality and activity limitation

Expected Expected years free of

Activity years of life activity limitation

Mortality limitation Years n1 Percent n2 Years n1 Percent n2

Observed3 ................ Observed3 75.8 ... ... 62.2 ... ...

5% reduction ............... 10% reduction 76.4 0.6 0.8 63.9 1.7 2.7

5% reduction 76.4 0.6 0.8 63.2 1.0 1.6

Observed3 76.4 0.6 0.8 62.5 0.3 0.5

5% increase 76.4 0.6 0.8 61.8 –0.4 –0.6

10% increase 76.4 0.6 0.8 61.1 –1.1 –1.8

10% reduction .............. 10% reduction 77.0 1.2 1.6 64.3 2.1 3.4

5% reduction 77.0 1.2 1.6 63.5 1.3 2.1

Observed3 77.0 1.2 1.6 62.8 0.6 1.0

5% increase 77.0 1.2 1.6 62.1 –0.1 –0.2

10% increase 77.0 1.2 1.6 61.4 –0.8 –1.3

. . . Category not applicable. 1n = difference (in years): Model estimate - Observed years. 2Percent n = percent difference between observed value and model estimates: n /Observed years. 3Observed values are from 1995.

health expectancy. However, healthy life, as a proportion of 

total life, is decreased when mortality declines. This occurs 

when there is no associated reduction in morbidity. The data 

presented here for the decade 1985 to 1995 portray a 

situation in which a decrease in mortality (8 percent) is 

associated with an increase in activity limitation due to 

chronic health conditions (5 percent). This situation is 

consistent with the observation that reductions in mortality 

are sometimes characterized by the survival of people in 

poor health who formerly would have died (13). Reducing 

morbidity, on the other hand, increases both the number of 

healthy years and the proportion of life that is healthy 

because reducing morbidity in isolation of any changes in 

mortality does not have any impact on life expectancy. 

These conclusions were true regardless of the age groups 

affected by the reductions in either mortality or morbidity. 

In this report, a number of models assuming simple 

proportional reductions for specified age groups were 

evaluated. Based on these evaluations, the improvements that 

can be achieved in limitation-free years are potentially much 

greater when activity limitation is reduced, as contrasted 

with mortality, because the morbidity rates are much higher 

than mortality rates at all ages. 

The results of the models presented here can be used to 

estimate the size of the reduction needed to increase years 

free of activity limitation. For example, consider an increase 

of 1 additional year free of activity limitation. To achieve 

such a gain, either a 12-percent reduction in mortality at all 

ages (figure 3), a 7-percent reduction in morbidity (figure 3), 

or a 5-percent reduction in both mortality and morbidity 

(table 5) would be needed. Similar analyses could be 

conducted using other measures of morbidity. For example, 

self-assessed health status is often used to evaluate morbidity 

because this measure is a global evaluation of health status. 

Self-assessed health status can be used to calculate the 

expected years of good or better health. Because the 

age-specific rates for fair or poor health (i.e., not ‘‘good or 

better’’) are generally less than for activity limitation, larger 

reductions (15 percent, data not shown) are needed to result 

in a 1-year gain in expected years of good or better health. 

The greatest increase in health expectancy can be 

derived by reducing morbidity and mortality concurrently. In 

reality, public health and medical programs to improve one 

aspect of health often yield some reductions in both 

mortality and morbidity. Consequently, the impacts on health 

expectancy estimated by reducing mortality alone probably 

underestimate the impact on health expectancy. 

Current and recent past statistics are a guide to the 

amount of change that might be reasonably expected for 

years of healthy life. Although the magnitude of 

limitation-free expectancy statistics for the total population 

has not changed substantially in the past decade (table 2), the 

potential to alter the years free of activity limitation is 

large (tables 3 and 4). Between 1985 and 1995, life 

expectancy increased 1.1 years. The improvement noted for 

expected years free of activity limitation, however, only 

increased 0.6 years during this same time period. This 

indicates that this gain in life expectancy was not associated 

with a similar gain in years of quality health. In fact, the 

proportion of life lived free of activity limitation only 

decreased by 0.4 percent, from 82.5 percent in 1985 

(61.6/74.7 years) to 82.1 percent in 1995 (62.2/75.8 years). 

Even without a future change in mortality, there remains an 

almost 14-year difference between total expected life years 

and expected years free of activity limitation that could be 

impacted with morbidity reduction. 

The improvement in health at one age might be 

correlated with improvements at later ages. This report did 

not evaluate subsequent changes in either mortality or 

morbidity resulting from hypothesized changes to specific 

age groups. One disadvantage with the life table method, as 

it is used here and in most situations, is that the age-specific 

rates are all evaluated at one time period. To evaluate the 

potential impact of a public health policy, one would want to 

10
consider subsequent changes to age-specific rates of 

mortality or morbidity. 

There are many considerations and social values that are 

important in the decision on how to allocate resources for 

public health programs. The sensitivity analyses do not 

address most of the considerations in that decision. The 

analyses presented here address the question of how 

reductions in mortality or morbidity produce the outcome of 

lengthened healthy life. The magnitude of the changes 

depend, however, on the measure of morbidity chosen. The 

higher the frequency of the morbid condition in the 

population, the greater the impact on healthy life expectancy 

of a proportional reduction in morbidity. 

Appendix

Calculating Expectancy Statistics

The health expectancy statistics are calculated using the 

life table technique as proposed by Sullivan (8). The life 

table is a statistical technique used by actuaries, 

demographers, public health workers, and others to describe 

the mortality experience of a population (14). In this 

technique, the expectation of life (ex) at any given age x is 

defined as: 

w 1 ex = 1x ∑ Li i = x, x+1,..., w i=x 

where 1x is the number of persons alive at exact age x, w is the 

maximum age, and Li represents the total number of personyears lived by a cohort in the age interval (i, i+1) (15). 

Expected years of healthy and unhealthy life from age x 

are derived by partitioning the estimated life expectancy 

at that age, based on the morbidity experience of the population (16,17). The average expected years of healthy life­

(HEx) and unhealthy life (UEx) at age x are defined as 

follows: 

w 1 

HEx = 1x ∑ (1 − pi

) Li i=x, x+1,..., w i=x 

and 

w 1 

UEx = 1x ∑ piLi i = x, x+1,...,w i=x 

where pi is the prevalence rate at age i of being in an 

unhealthy state of health. 

Estimating Morbidity

The 1985 and 1995 activity limitation estimates were 

calculated by combining data from the National Health 

Interview Survey (for the prevalence of any activity 

limitation in the noninstitutionalized population) and 

National Nursing Home Survey (for the nursing home 

population size). For this analysis, all the nursing home 

residents in 1985 and 1995 are assumed to be limited in 

activities. Therefore, for each age group, the percent of the 

total population with activity limitation is estimated as: 

( f v NNI + NNH)/(NNI + NNH) 

where f is the age-specific prevalence of any activity limitation in the noninstitutionalized population, NNI is the agespecific size of the noninstitutionalized population, and NNH 

is the age-specific size of the nursing home population. 

Sources of Data

Population Estimates Used to Calculate

Mortality

The national population estimates used to calculate the 

mortality rates are derived by using the decennial census data 

as benchmarks and data available for interim years on births 

and deaths, immigrants, Armed Forces, net movement between 

Puerto Rico and the U.S. mainland, and Federal employees 

working abroad (12). These data include institutionalized 

and noninstitutionalized residents of the United States. 

The data are available from the U.S. Bureau of Census at 

http://www.census.gov/population/www/estimates/uspop.html. 

Death Statistics

The number of deaths by age to residents of the 

United States are available through the National Vital 

Statistics System, which is maintained by the National 

Center for Health Statistics, Centers for Disease Control 

and Prevention (18). More information is available at 

http://www.cdc.gov/nchs. 

National Health Interview Survey

The National Health Interview Survey (NHIS) was used 

in the estimation of morbidity in the noninstitutionalized 

population. NHIS, conducted by the National Center for 

Health Statistics, Centers for Disease Control and 

Prevention, is a continuing nationwide sample survey in 

which data are collected through personal household 

interviews (19). NHIS data have been collected continuously 

since 1957. The data in this report were collected during the 

surveys in 1985 and 1995. Information is obtained about the 

health and other characteristics of each member of the 

household, including data on personal, sociodemographic, 

and health characteristics of the family members and 

unrelated individuals living in these households. The sample 

design permits estimation of personal and health 

characteristics of the civilian noninstitutionalized population. 

More information is available at http://www.cdc.gov/nchs. 

National Nursing Home Survey

The National Nursing Home Survey (NNHS) was used 

to estimate the size of the nursing home resident population 

in 1985 and 1995. For this analysis, all residents were 

assumed to be limited in activities. The NNHS is a periodic 

nationwide sample survey, conducted by the National Center 

for Health Statistics, Centers for Disease Control and 

Prevention, of nursing homes, their residents, discharges, and 

staff. 

11
The NNHS uses a two-stage probability design. The first 

stage selection is a probability sample of the nursing 

facilities, and the second stage is the selection of a sample 

of current residents from a list of all residents who are on 

the rolls of the facility as of midnight the day before the 

survey. Facilities included in the NNHS are nursing and 

related-care homes having three beds or more set up and 

staffed for use by residents and routinely providing nursing 

and personal care services. Data collected about current 

residents include basic demographic, living arrangement 

prior to admission, functional status, and other health and 

personal characteristics of the residents. The data on 

residents are collected by interviewing knowledgeable 

nursing home staff members. Estimates of the total resident 

population can be obtained by using the resident population 

weights (POPRES) that account for both stages of the 

sampling procedure. The 1985 survey had a sample size of 

5,243 residents (20), and the 1995 survey had a sample size 

of 8,050 residents (21). More information is available at 

http://www.cdc.gov/nchs. 

Figure Data

Tables I and II show data used in figures 1–3. Table I 

displays age-specific death rates for 1900 and 1995 

separately by gender. Table II displays age-specific death 

rates and the percent of the population reporting any activity 

limitation due to chronic health conditions for 1995. 

References

1.	 National Center for Health Statistics. Health, United States, 

2000, With adolescent health chartbook. Hyattsville, Maryland. 

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2.	 U.S. Department of Health and Human Services. Healthy 

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health and objectives for improving health. 2 vols. 

Washington: U.S. Government Printing Office. November 

2000. 

3.	 Institute of Medicine. Summarizing population health: 

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metrics. Field MJ, Gold MR (eds.) Washington: National 

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4.	 Fryback D. Health-related quality of life for population health 

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development and application of population metrics. Field MJ, 

Gold MR (eds.) Washington: National Academy Press. 1998. 

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6.	 Erickson P, Wilson R, Shannon I. Years of healthy life. 

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7.	 National Center for Health Statistics. Healthy People 2000 

review, 1998–99. Hyattsville, Maryland: Public Health Service. 

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8.	 Sullivan DF. A single index of mortality and morbidity. 

HSMHA Health Reports 86 (4):347–354. 1971. 

Table I. Death rates per 100,000 resident population, by age and

sex: United States, 1900 and 1995

Male Female

Age 1900 1995 1900 1995

All ages, crude .............. 1,791.1 914.3 1,646.9 846.9

Less than 1 year ............. 17,914.4 852.5 14,541.2 695.6

1–4 years ................. 2,054.2 44.9 1,912.3 36.3

5–14 years ................ 384.3 260.9 387.5 18.2

15–24 years ............... 594.0 139.4 577.6 47.9

25–34 years ............... 824.3 206.1 815.2 77.9

35–44 years ............... 1,067.7 332.5 975.2 149.8

45–54 years ............... 1,569.9 598.5 1,417.7 327.5

55–64 years ............... 2,874.1 1,417.8 2,576.0 840.5

65–74 years ............... 5,929.0 3,284.6 5,358.3 1,987.2

75–84 years ............... 12,826.4 7,352.7 11,877.3 4,870.7

85 years and over ............ 26,877.2 17,501.2 25,517.4 14,338.2

SOURCE: Centers for Disease Control and Prevention, National Center for Health Statistics.

National Vital Statistics System.

Table II. Death rates and any activity limitation due to chronic

health conditions, by age: United States, 1995

Total Any activity

Age mortality limitation1

Less than 1 year ............. 8 13

1–4 years ................. 0 30

5–9 years ................. 0 64

10–14years ................ 0 79

15–19 years ............... 1 79

20–24 years ............... 1 69

25–29 years ............... 1 81

30–34 years ............... 2 97

35–39 years ............... 2 118

40–44 years ............... 3 140

45–49 years ............... 4 169

50–54 years ............... 6 217

55–59 years ............... 9 262

60–64 years ............... 4 311

65–69 years ............... 1 366

70–74 years ............... 31 308

75–79 years ............... 47 381

80–84 years ............... 75 480

85 years and over ............ 152 657

1Activity limitation includes both the frequency of activity limitation reported by the

noninstitutionalized population and the proportion of the population in nursing homes, who

are all assumed to have activity limitation.

NOTE: Rates are shown as number per 1,000 population.

SOURCES: Centers for Disease Control and Prevention, National Center for Health

Statistics. National Vital Statistics System (mortality); National Health Interview Survey

(noninstitutionalized population); National Nursing Home Survey (nursing home population).

9.	 Molla M, Wagener DK, Madans JH. Summary measures of 

population health: Methods for calculating health expectancy. 

Healthy People Statistical Notes No. 21. Hyattsville, 

Maryland: National Center for Health Statistics. 2001. 

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morbidity rates and the health status and life expectancy of the 

older population. Demography 31(1):159–175. 1994. 

11. Crimmins E, Hayward M, Saito T. Differentials in active life 

expectancy in the older population of the United States. 

J Gerontology: Social Sciences 51B(3):S111–S120. 1996. 

12.	 U.S. Bureau of the Census. U.S. population estimated by age, 

sex, race, and Hispanic origin: 1990–96. Release PPL-57. 

March 1997. 

12
13.	 Gordis L. Epidemiology. 2nd ed. Philadelphia, Pennsylvania: 

WB Saunders. 2000. 

14.	 Namboodiri K, Suchindran CM. Life table techniques and their 

applications. Orlando, Florida: Academic Press. 1987. 

15.	 Jagger C. Health expectancy calculation by the Sullivan 

method: A practical guide. NUPRI Research Paper Series 

No. 68. Tokyo, Japan. 1999. 

16.	 Keyfitz N. Applied mathematical demography. 2nd. ed. New 

York, NY: Springer-Verlag. 1985. 

17.	 Mathers CD. Health expectancy in Australia 1981 and 1988. 

Technical Report. Australian Institute of Health: AGPS, 

Canberra. 1991. 

18.	 National Center for Health Statistics. Technical appendix, Vital 

statistics of the United States, 1992. Vol II. Mortality. DHHS 

Selected published issues of Healthy People Statistical Notes

Pub. No. (PHS) 96–1101. Washington: U.S. Government 

Printing Office. 1996. 

19. Bensen V, Marano MA. Current estimates from the National 

Health Interview Survey, 1995. National Center for Health 

Statistics. Vital Health Stat 10(199). 1998. 

20.	 Hing E. Nursing home utilization by current residents: United 

States, 1985. National Center for Health Statistics. Vital Health 

Stat 13(102). 1989. 

21.	 Gabrel C, Jones A. The National Nursing Home Survey:1995 

Summary. National Center for Health Statistics. Vital Health 

Stat 13(146). 2000. 

Number Title Date of Issue

Healthy People 2000

2 Infant Mortality Winter 1991

6 Direct Standardization (Age-Adjusted Death Rates) March 1995

7 Years of Healthy Life April 1995

19 Healthy People 2000: An Assessment Based on the Health Status November 2000

Indicators for the United States and Each State

Healthy People 2010

20 Age Adjustment Using the 2000 Projected U.S. Population January 2001

21 Summary Measures of Population Health: Methods for August 2001

Calculating Healthy Life Expectancy

NOTE: These issues of Healthy People Statistical Notes deal with statistical issues affecting the tracking of Healthy People 2010.

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Suggested citation

Wagener DK, Molla MT, Crimmins EM, Pamuk E,

Madans JH. Summary measures of population

health: Addressing the first goal of healthy people

2010, improving health expectancy. Statistical Notes,

no. 22. Hyattsville, Maryland: National Center for

Health Statistics. September 2001.

DHHS Publication No. (PHS) 2001-1237

1-0510 (9/01)