A longitudinal study is a research method that involves a sample of data being collected repeatedly for the same individuals over time, possibly for many years.
Types of longitudinal studies include:
- cohort studies - where a group of people with a shared defining characteristic are observed at certain intervals over a period of time; and
- panel studies - where individuals are followed over time, with repeated measures being taken from the same sample at different intervals.
The ONS Longitudinal Study differs from the above types because its sample was initiated at the time of the 1971 Census and four birth dates have since been used to update the sample at the 1981,1991 2001 and 2011 Censuses and in routine event registrations. Thus, individuals enter the study through birth and immigration and leave through death and emigration so that the LS represents a continuous sample of the population of England and Wales, rather than a sample taken at one time point only.