by Nicola Shelton

Back in the late 1960s there was concern that policymakers had too little information about births and deaths: death certificates recorded only limited information and even the occupation of the deceased could be recorded inconsistently. Similarly it was impossible to use information from birth registrations to look at patterns of fertility – how were children spaced within families, for instance? And so the ONS Longitudinal Study was born.

The 1971 Census had recorded respondent’s date of birth – as opposed to age –  for the first time. And that allowed statisticians to record data on a one per cent sample of the population – all those born on four dates of birth which were, and remain, a closely-guarded secret.

The ONS Longitudinal Study now holds records for more than a million people, none of whom have any idea that they are a part of the study. It’s only possible to join through being born in the UK or through migrating into it, and it’s only possible to leave by dying or emigrating. The study also holds information for those living with its members, but it doesn’t follow them up in the same way from census to census.

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.

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