On the measurement of expected lifespan

Does the measured expected lifespan accurately describe the actual expected lifespan? We will discuss some extreme cases to shed light on this important issue.

Suppose a group of young people, all aged thirty, migrate to an uninhabited island. The first year, one or two persons died of illness or accident. The measured expected lifespan, by definition, is thirty years old. The second year, some more people died. The expected lifespan becomes 31 years. Then 32, 33, 34, … Suppose this group of people don’t have any offspring. No one else migrated to the island later. When the last person from this group died at 110 years old, the expected lifespan is 110 years old.

When all people of this group die off, we can calculate the actual lifespan of this group by averaging all lifespans. It may be 80 years. But the expected lifespan of this group of people increases from 30 to 110, depending on the year of the measurement.

Next, we will discuss another case. In a remote area, the expected lifespan has been steady at 60 years for a long period of time. In 1930, electricity was introduced into this region. The convenience greatly improved the quality of life for the people in this area. From the statistics conducted in 2020, people born after 1930 had an average lifespan of 80 years. Does the expected lifespan, measured every year after 1930, become 80 years? No. The measured expected lifespan will only rise slowly and steadily from 60, reaching 80 after many decades.

Governments like to interpret the steady increase in the expected lifespan as the steady improvement of the society. Researchers, most of whom are sponsored, directly or indirectly, by the government, tag along. They often produce sophisticated looking statistical methods to intimidate anyone who might question the official interpretations.

When the actual lifespans stabilize, or even decline, the measured expected lifespan may continue to increase for a long time.

We create various measurements to help us understand the world. But we often forget that each measurement has its own limitations. In the end, we are often misled by these measurements. Authorities, who design, modify and interpret these measurements, often guide the understanding of the public in certain ways that make the authorities look good.

P.S. I wrote a similar note on the measurement of total fertility rate. The link to the note is listed below. The time lag on expected lifespan may be longer than total fertility rate.

https://blog.creaders.net/u/10630/201910/357688.html