The life expectancy figures estimate the average number of additional years a man or woman aged 75 can be expected to live if they continue to live in the same place and the death rates in their area remain the same for the rest of their life. Figures are based on their year of birth, current age (in this case at age 75) and other demographic factors including their gender. Life expectancies are calculated using life tables which show, for each age, what the probability is that a person will die before his or her next birthday.
Expectations of life can be calculated in two ways: ‘period life expectancy’ or ‘cohort life expectancy’.
Period life expectancies use mortality rates from a single year (or group of years) and assume that those rates apply throughout the remainder of a person’s life. This means that any future changes to mortality rates would not be taken into account. In practice, death rates are likely to change in the future so period life expectancy does not therefore give the number of years someone could actually expect to live. Also, when looking at specific geographical areas, the method makes no allowance for the different areas people may live in for at least some part of their lives which may have different mortality rates.
However, period life expectancies are used in this indicator because they provide a useful measure of mortality rates actually experienced over a given period and, for past years, provide an objective means of comparison of the trends in mortality over time, between areas of a country and with other countries.
A cohort life expectancy is calculated using a combination of observed mortality rates for past years and projections about mortality rates for a group of people born in a certain year in future years. Cohort life expectancies are thus regarded as a more appropriate measure of how long a person would be expected to live than period life expectancies. Cohort life expectancies are commonly found in ONS (Office for National Statistics) publications.