A birth cohort is a group of people from a given population which is defined by the time of their birth. For example, the
group of men born in the UK in 1930 form a birth cohort (the given population is men born in the UK and the birth definition
is 1930), whilst the group of men born in the UK over the period 1925 to 1935 also form a (wider) birth cohort (the given
population is unchanged, but the birth definition is now 1925 to 1935). The term is often abbreviated to just ‘cohort’; but
cohorts can be equally well analysed according to year of retirement or year of starting employment.