Demographic cohort
= Demographic cohort
A <demographic cohort> is a group defined by experiencing a specified event during the same interval, then followed through time. A birth cohort consists of people born in a stated period; other examples group people by year of immigration or first diagnosis. It is not the same as all people of a given age in a single cross-section.
Ignoring migration, a birth cohort with 1000 initial members and 950 survivors ten years later has ten-year survival proportion $950/1000=0.95$. Comparing that with the proportion of today's ten-year-olds among today's residents answers a different question, because births, deaths and <human migration> have affected the denominator. Cohort analysis separates changes experienced by a group as it ages from changes between successive groups. For a cohort defined by birth year $c$, calendar year and age satisfy $t=c+a$, which creates an exact dependence in attempts to estimate age, period and cohort effects simultaneously.