Period and cohort life expectancy

What is the difference between two measures of life expectancy?

How long do you expect to live? That is a key question which affects financial planning and health decisions. Life expectancy is a statistical measure of how long a person expects to live. That expectancy depends upon demographic factors, like age and sex.

There are two main ways to think about life expectancy:

  • Cohorts: the mean lifespan of people born in specific year (or set of years). This calculation uses observed mortality and projections.
  • Period: calculations using mortality rates from a year (or years), turn into average years of life. That assumes those rates stay constant throughout each person’s life.

Cohort life expectancy

Researchers calculate life expectancy from life tables. Cohort life tables show the chance of someone in an age cohort dying in specific years of their life. These tables use observed mortality during past years, and projections for future years.

Suppose we wanted to calculate the cohort life expectancy of those aged 60 years old in 2015. Analysts would use death rates of people aged 60 in 2015, aged 61 in 2016, aged 62 in 2017, and so on.

Once we get into future years, we start using projections. Considering the future introduces inherent uncertainty and subjectivity into the calculations.

Different projections lead to different results. (Image: Office for National Statistics)

Period life expectancy

Period life expectancy is then applying mortality rates in a single year (or years) to a group of newborns. The calculation assumes there will be no mortality changes in future. The probability of proceeding from 59 to 60 is fixed and applies to all.

It is another way of expressing mortality rates. In a year where there was a large increase in mortality, period life expectancy for 2020 in England fell:

Period life expectancy fell in England in 2020. (Image: Public Health England)

Period life expectancy is comparable within and across countries.

Why do these two types of life expectancy differ?

Period life expectancy tends to be lower than cohort life expectancy. That is because there is a general reduction in mortality rates over time.

Period life expectancy calculations do not account for these improvements. That kind of expectancy represents fixed periods in time, not futures yet to come.

The Office for National Statistics produced life expectancy projections, from 2018 in England and Wales. Cohort life expectancy was around seven to eight years higher than period expectancy. That is using 2018-based projections.

Here, cohort life expectancy is higher than period expectancy. (Image: Office for National Statistics)

Cohort and period life expectancy calculations are not the same. Average life expectancy at birth differs to life expectancy at later ages too.

This blog looks at the use of statistics in Britain and beyond. It is written by RSS Statistical Ambassador and Chartered Statistician @anthonybmasters.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store