Definition
A cohort life table is a mortality table constructed by following an actual birth cohort across its lifetime, recording the mortality experience that the cohort itself realized at each age it passed through.
Why it matters
Cohort life tables are the empirical record of what actually happened to a generation of people, in contrast to period tables, which describe what mortality conditions looked like at a single moment. They are the truer measure of any specific cohort's lifetime mortality, but they are only fully complete for cohorts whose members have all died — which restricts complete cohort tables to generations born long enough ago that direct planning use is limited.
How it works
A cohort life table follows a specific birth cohort — for example, US females born in 1920 — across each year of that cohort's life, recording the mortality rate the cohort actually experienced at each age. The age-65 mortality rate in such a table is the rate the 1920 cohort experienced in 1985, when they reached 65; the age-85 rate is the rate they experienced in 2005, and so on. Constructed this way, the cohort table captures the actual mortality improvement the cohort lived through, because the rate at each age reflects the calendar-year conditions that prevailed when the cohort was that age. Complete cohort tables exist only for cohorts that have effectively all died, which today means cohorts born no later than the late nineteenth century. For more recent cohorts, the early-age portion of the table is observed and the later-age portion is projected, which blends the cohort table into a projected mortality table. Cohort tables for fully-extinct historical cohorts characteristically show substantially higher life expectancy at 65 than the period tables in use when those cohorts were 65, because mortality improved during the cohort's later life.
In practice
For an individual planning their own retirement income, the cohort table for their birth year — though it cannot yet be fully observed — is conceptually the right reference, because it would describe the mortality conditions they will actually live through. Projected mortality tables approximate this by combining a period base with a mortality improvement scale; the projected table for an individual's birth year is the practical substitute for the cohort table that does not yet exist. A professional explaining a lifetime income illustration's planning horizon assumption is implicitly working with either an explicit projected table or an implicit cohort interpretation, and asking which is reasonable.
In the Longevity Standard Framework
A cohort life table is supporting vocabulary in the Longevity Standard framework, distinguished from period life tables by its longitudinal construction and from projected mortality tables by its empirical rather than assumed treatment of mortality improvement. The distinction between period, cohort, and projected mortality is operationally consequential — solo drawdown's analytical baseline, the frictionless pool's pricing, and the realized value calculation all depend on the mortality reference chosen.
Related terms
- Period life table
- Mortality table
- Projected mortality table
- Mortality improvement
- Cohort effect
- Life expectancy
- Survival curve
- SOA mortality research