Contains migration flows within and between wards in the UK. The set of wards used are as follows:

• Census Area Statistics ('CAS') wards in England and Wales
• Standard Table ('ST') wards in Scotland and Northern Ireland

ST and CAS wards are identical for the most part; however in a few cases small CAS wards have been merged with larger neighbours in order to satisfy output thresholds. No such merging was necessary for Northern Ireland, so there is no distinction between ST and CAS wards there.

The mixture of wards is unique to the 2001 interaction data, and the set have therefore been referred to as interaction data wards.

All the 2001 SMS data have been derived from the 100% sample and include imputed households. Where migrant origins were unknown, these were also imputed.

Disclosure control

Data associated with destinations in England, Wales and Northern Ireland have been modifed via a process known as Small Cell Adjustment Methodology (SCAM). The SCAM process is a disclosure control mechanism. For all tables process using SCAM, interior cells with a 'small' value have been adjusted. 'Small' values are understood to be in the range 0-3. Cells with an initial value of 1 have been rounded to either 0 or 3, with 0 being the more likely result. Cells with an initial value of 3 have also been rounded to either 0 or 3, but with 3 being the more likely result. Cells with initial values of either 0 or 3 have retained these values although in each case it is impossible to distinguish between rounded values and 'genuine' 0s or 3s. All sub-totals and totals have been re-calculated from the rounded cells.

There are three noticeable effects of SCAM

1. Within-table modification. For the majority of outputs, it will be apparent that a large proportion of cells within any table are either 0 or 3, and that sub-totals and totals within the table are multiples of 3.
2. Between-table variation. SMS Levels 1 and 2 contain several output tables, with the result that a number of values (e.g. 'total migrants') can be found in more than one table. Due to SCAM, these values may not be consistent between tables.
3. Between-level variation. The three spatial levels for which the SMS are available were independently subject to SCAM. Thus, if a particular value (e.g. 'total migrants') is calculated by aggregating from one spatial level to a higher one, and compared to the value found in the SMS data set for that higher level, those values are unlikely to be consistent.

Users are advised to ensure that data they use has been calculated by summing the minimum number of base values.