Conventional demographic measurements are either period or cohort in type. The latter is perhaps the simpler in conception. Cohort measures relate to the experience of a group of individuals over a whole or a part of their lifetimes. Thus, a cohort measure of expectation of life at birth is obtained by accumulating data relating to a group of individuals all of whom were born in the same year, quinquennium, or other time period. For example, 100000 children might be born in a given country in the year 1700. All will have died by the year 1800 or shortly thereafter. The period of time from which data are taken for a cohort calculation of this sort, therefore, will span a century. If the age at death of each individual is known, mean expectation of life at birth can readily be calculated.

Alternatively, a period measure of expectation of life for the year 1700 may be obtained by securing data from the many cohorts which were all in existence in that year and using them to construct an ‘artificial’ cohort. This exercise will show what expectation of life would have been if the mortality levels prevailing in each age group in 1700 had continued to hold true long enough for a cohort born in that year to have been completely exhausted by mortality. Some of the cohorts involved in the construction of the period rate will, of course, have been born 80 years or more before the children whose success or failure in surviving their first year of life will provide the data needed to calculate an infant mortality rate for the year 1700 itself.