The 1981 SMS refer to counts of individuals who were in existence 12 months before the Census and who were resident at a different usual residence on 5 April 1981 to that a year previously.

There are two data sets. 1981 SMS Set 1 are a complex set of flows within wards, from each ward to each district and vice versa, from outside GB to each ward and from origin-not stated to each ward. Cole and Squires (1987) explain how the structure of the data is complicated further by 'thresholding' procedures to ensure confidentiality which greatly limit use of the data.

1981 SMS Set 2 are separate counts of males and females migrating within and between each pair of wards defined in 1981. They have not been subject to data adjustment or thresholding. In an attempt to create a consistent comparison between 1981 and 1991, a recent Census Development Project has involved the re-estimation of 1981 SMS Set 2 data for wards whose boundaries were used in the 1991 Census. The estimation methodology is available in a paper published in Computers, Environment and Urban Systems (Feng and Boyle, 2001)

Users can choose between 2 counts (males and females) in the re-estimated SMS Set 2 data for which the geographical areas are identical to those used in 1991.

References

Ballard, B. and Norris, P. (1983) User needs - an overview, Chapter 3 in Rhind, D. (ed) A Census User's Handbook, Methuen, London: 89-113.

Cole, K. and Squires, S. (1987) The Special Migration Statistics from the 1981 Census: the data and their implications, Paper presented at the LAMSAC Workshop, County Hall, London.

Feng, Z. and Boyle, P. (2001) Integrating the 1981 and 1991 census interaction data, Computers, Environment and Urban Systems, forthcoming.