Contains migration flows within and between output areas in the UK.

Users can choose from 12 counts of migrants in 1 table. This set of 12 counts includes a number of totals and sub-totals; the number of interior cells which form the basis of these sub-totals is 6.

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.