Q: Your system has given me trends from 1801 for the local authority I live in, but I know it was only created in 1996 (or 1974, or 1965). How can you do this?

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A: Our statistics for 2011 come straight from the census, but all our earlier statistics for "modern" units - counties and regions as well as districts - are estimates. However, we believe they are pretty good estimates:

  • Our data for 2001, 1991 and 1981 are very detailed Small Area Statistics, generally from the NOMIS system. Many local authorities did not change their boundaries between these censuses and 2011, but where the boundaries are different we have used our GIS software and digital boundary data to redistrict from wards or Super Output Areas to 2011 districts.
  • For earlier dates, we have to work with the published statistics which were generally for some kind of district, mainly Registration Districts between 1851 and 1911, and Local Government Districts between 1911 and 1971. There are usually more historical districts than modern ones - for example, 1,800 Local Government Districts in England and Wales in 1931, versus 376 today - but the historical districts do not fit neatly into the modern ones. What we have done uses all our information on boundaries, both modern and historical, and also our information on the historical population of parishes. Assuming that people were evenly spread across each parish, we work out what proportion of the population of each historical district fell within the boundaries of each modern district. We then assign that proportion of each category in the district-level historical statistics - people unemployed, people aged 20 to 24, and so on - to the modern district. This of course involves a second assumption: that the proportion of all people who fell into the category was constant across the historical district.
  • Both our assumptions are obviously not entirely true, but alternative methods are complex and would be very hard to explain here. They also make just as big assumptions. Fortunately, most of the time this does not matter: all, or almost all, of each historical unit goes into just one modern unit. This is particularly true of cities: the modern units are bigger than, and completely surround, the historical ones. There are a few places where we have used the only available historical data to give you some idea of past patterns, even though there are fewer historical units than modern ones; for example, where the only Scottish data is for counties and the four main cities. Our accompanying text should warn you about this.
  • Of course, you may feel this whole approach is misleading: presenting 1801 or 1851 data using the boundaries of modern cities means that most of the area covered was then open farmland. One answer is that most of the people within that area would have been living within the built-up area. Another answer is that we usually be able to also give you statistics for the Borough or Urban District that existed back then.
  • The divide between pre-1974/5 historical units and the "modern" units we provide re-districted data is quite straightforward. We do not hold information on the evolution of administrative units between 1974/5 and the present. However, the modern London Boroughs were created in 1965, not by the big reform in 1974, so they exist in the system twice, as Local Government Districts whose boundaries may change slightly between 1965 and 1974, and as "Modern Districts" which we trace back to 1801.
  • Parliamentary Constituency data is the exception to the norm, with boundary changes covering the whole period from 1832 to now.

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