What is the ONS Longitudinal Study?
What is the LS?
The ONS Longitudinal Study is a complete set of census records for individuals, linked between successive censuses, together with data for various events. It relates to a sample of the population of England and Wales.
The sample comprises people born on one of four selected dates of birth and therefore makes up about 1% of the total population. The sample was initiated at the time of the 1971 Census, and the four dates were used to update the sample at the 1981,1991 and 2001 Censuses and in routine event registrations. New sample members found in the 2011 Census will be added to the sample. Fresh LS members enter the study through birth and immigration and existing members leave through death and emigration.
Thus, the LS represents a continuous sample of the population of England and Wales, rather than a sample taken at one time point only. It now includes records for over 950,000 study members.
In addition to the census records, the individual LS records contain data for events such as deaths, births to sample mothers, emigrations and cancer registrations.
Census information is also included for all people living in the same household as the LS member. However, it is important to emphasise that the LS does not follow up household members in the same way from census to census.
Why was it set up?
The LS was planned in the late 1960s at a time of considerable concern about the adequacy of mortality data collected from death registrations, and about the lack of data on fertility patterns. For example, when a death is registered, only very limited socio-economic information can be collected about the dead person, some of which (e.g. occupation) may be either deficient or inconsistent with that collected while the person was alive. More reliable and more extensive statistics could be obtained by linking the death record with the earlier census return for that person.
Similarly, the information provided on birth certificates did not allow for studies of birth spacing. Although such data could be obtained from the General Household Survey (GHS), the total sample sizes were too small for detailed studies.
The 1971 Census was the first to include a question on date of birth (rather than age), and this, combined with advances in information technology, made the LS possible.
What types of analysis is it used for?
- Prospective analysis of census and event data. For example: studies of associations between employment status and mortality, between economic status and cancer registrations, or between socio-economic factors and fertility; survival analysis of mortality by area deprivation.
- Prospective analysis of successive census data. For example: analysis of patterns of retirement migration, studies of the effects of divorce and remarriage on housing tenure, comparison of changes in education, employment and migration between the 1970s and 1980s.
- Prospective analysis of successive event data. For example, studies of changes in birth spacing, associations between fertility and cancer survival.
- Cross-sectional analysis, especially for studies of ethnic or geographical factors.
- International comparisons. Countries such as France, Denmark, Finland and the USA have similar programmes to the LS.
Further details of studies that have been undertaken using the LS are detailed in What has the LS been used for?.