A life table is a table giving information on the survivorship rates at each age (i.e., the probability of survival from an age of years to an age of years, for each integer value of ). Life tables are of two kinds.
Period life table (also known as static life table)
A period life table, also called a static life table, is a life table where the survivorship probabilities for all ages are based on data collected in a single period (usually a year). Thus, a period life table for the year 1996 gives probabilities of survival at each age in 1996, i.e., it gives the probability that a person aged 40 in 1996 will go on to age 41, and also separately gives the probability that a person aged 60 in 1996 will go on to age 61.
A period life table cannot directly be used to compute life expectancy or probability of survival over a longer time than one period, unless we assume that the survivorship does not change over time.
Cohort life table (also known as dynamic life table)
A cohort life table provides information on the survivorship probabilities for a given cohort (by birth) at each age. For instance, a cohort life expectancy for the cohort born in 1970 will include the probability that people in that cohort will survive from age 25 to age 26, and separately that people in that cohort will survive from age 45 to age 46. Cohort life tables can be used to estimate life expectancy at birth as well as life expectancy for the parts of the cohort that survive to specific ages.
Combining cohort life tables for different birth cohorts presents a comprehensive picture of population dynamics.
Cohort life tables can includes estimates for survivorships in future years. These estimates can be made in various ways, and are subject to debate.