Who Are We - Life Expectancy Basics

Life expectancy has been on a steady increase throughout the last century. In 1900, life expectancy at birth was just a shade over 47 years. In 1998, it reached nearly 77 years.

This life-affirming indicator of general well-being and social progress is closely linked with — indeed based upon — the death rate. It was 17.2 deaths per 1,000 population in 1900 and 8.6 in 1998. In the graphic above, Life Expectancy is shown on top, and total deaths (in thousands, right scale) are shown below. Note the sharp dip in 1918 — when life expectancy dipped to its lowest level in the 20th century (39 years) and deaths peaked; the death rate that year, 18.1 per 1,000 population (not shown), was also the highest in the century. More comment will follow below.

Life expectancy is calculated by looking at age-specific death rates in any given year. The death rate is simply the number of people who die per 1,000 population. These data are categorized by age. The rates are applied to a hypothetical population of 100,000 newborn babies. Some die in infancy. They are removed from the hypothetical population. The rest "go on living," but at each age some die, of course. The number used is determined by the measured rate taken from registries of death. Thus individuals are removed from the population at each age, using the age-specific death rates prevailing in the year of calculation. Years actually lived by each individual are cumulated. In 1997, for example, the death rates for that year indicated that 100,000 newborns would, altogether, live 7,650,789 years. This number, divided by 100,000, produces 76.5 — which was the life expectancy at birth, in years, for 1997.

This means that, in effect, life expectancy is not something achieved once and for all and "locked in" for all time. The measure is simply another way of depicting the mortality rate in a given year. This is illustrated by the data for the year 1918. The cause of the dip, that year (and in the spike in deaths) was not World War I but the so-called "Spanish Influenza." The sickness extracted some 20 million lives across the globe — and nearly 500,000 in the United States. For a while, after that, children sang a little ditty:

I had a little bird
And its name was Enza
I opened the door
And in-flew-Enza.

The death rates of children and young adults were especially affected in 1918 — as shown in the following table.

Increase in Death Rate, 1917 to 1918, in %

Age	Male	Female
<1	51.4	57.9
1-4	217.7	264.7
5-14	560.6	716.3
15-24	1273.7	1659.6
25-34	1438.1	1924.4
35-44	428.4	551.4
45-54	118.7	164.4
55-64	34.2	37.8
65-74	7.8	2.5
75-84	-18.7	-23.7
>84	-30.3	-29.8

Increases of more than 1,000% in the death rate produce dramatic results in life expectancy. But note that, in 1919, life expectancy had climbed again. This brings home the character of the life expectancy measurements — which are based on the death rates in a single year.

Note that, in the first half of the 20th century, life expectancy seesaws a good deal. These were times before modern medicine and sanitation took hold; tuberculosis stalked the land. After about World War II, life expectancy — and also total deaths — show a less dramatic up-down fluctuation.

We look at more detail on this interesting subject in the following three panels.

Source: U.S. Department of Health and Human Services. National Center for Health Statistics. National Vital Statistics Reports. Vol. 47. No. 28. December 13, 1999. Online. 2002. Available: www.cdc.gov/nchs/data/nvsr/nvsr47/nvs47_28.pdf.