The response from CIDS to the latter problem has been to construct a 5 tier hierarchy of zones from the initial set of 459 districts in 1991 and 426 districts in 2001:
- ‘district’ geography (417 zones)
- ‘intermediate’ geography (219 zones)
- ‘100 zone’ geography (100 zones)
- ‘city region’ geography (47 zones)
- ‘Government Office Regions’ (12 zones)
Data for these zone systems at supra-district scales are approximately consistent between 1991 and 2001
CIDS also re-estimated various sets of sub-district flows from 1981 and 1991 Censuses for 2001 boundaries (see Boyle and Feng, 2003)