The 1991 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 21 April 1991 to that a year previously.

MIGPOP data are counts derived from a subset of SMS Set 2 containing migration flows within and between districts in Great Britain. There were 459 districts in 1991 (366 in England, 37 in Wales and 56 in Scotland).

All the 1991 SMS data have been derived from the 100% sample and include imputed households. However, OPCS suppressed certain migrant data in the SMS Set 2 in order to ensure confidentiality. Tables 1 and 2 contain unsuppressed counts. However, the imposition of suppression thresholds means that many smaller flows are not available in Tables 3-10. Phil Rees and Oliver Duke-Williams (1997) have estimated data for suppressed flows using various estimation methods.

Ludi Simpson and Erica Middleton identified a number of further elements missing from the SMS data and have adjusted the flows corrected for suppression by Rees and Duke-Williams in Table 3 with the 15-19 year olds as a single group. The estimation for underenumeration methodology and its implications are outlined in a paper in the International Journal of Population Geography (Simpson and Middleton, 1999).

Users can choose from 36 counts in Table 3 of migrants by age group and sex.

References

Flowerdew, R. and Green, A. (1993) Migration, transport and workplace statistics from the 1991 Census, in Dale, A. and Marsh, C. (eds) The 1991 Census User's Guide, HMSO, London: 269-94.

Rees, P.H. and Duke-Williams, O. (1995) The story of the British Special Migration Statistics, Scottish Geographical Magazine, 11: 13-26.

Rees, P.H. and Duke-Williams, O. (1997) Methods for estimating missing data on migrants in the 1991 British Census, International Journal of Population Geography, 3: 323-368.

Simpson, S. and Middleton, E. (1999) Undercount of migration in the UK 1991 Census and its impact on counterurbanisation and population projections, International Journal of Population Geography, 5: 387-405.