Table C0711 was created as commissioned output from the 2001 Census. For ease of use it has been split into three parts:

• C0711A - Migrants by age and ethnic group
• C0711B - Migrants from outside UK by ethnic group
• C0711C - Migrants from 'no usual address' by ethnic group

C0711A has been modified by CIDER. The original C0711 table was created by the Office for National Statistics, and related to destinations in England and Wales only. Equivalent data were commissioned from the General Register Office for destinations in Scotland, and have been added to the C0711A data set.

Note Users of the data should be aware of the geographic scope of this data set, as it is not the same as most other 2001 Census data sets hosted by CIDER. The modified C0711A data set contains the following flows:

• From Local Authorities in the UK to Local Authorities in England and Wales
• From Local Authorities in Great Britain to Local Authorities in Scotland

Thus, whilst there are tabulated flows from origins in Northern Ireland to destinations in England and Wales, there are no such flows to destinations in Scotland, and no flows to any destinations in Northern Ireland from any origin.

Finally, users are advised that as an additional disclosure control mechanism (see below), flows to the City of London have been merged into counts for flows to Westminster, and similarly flows to the Isles of Scilly have been merged into those for Penwith district.

The 2001 SMS Level 1 data include flows disaggregated by ethnic group for a comprehensive UK geography, but without the additional age break down included in the C0711A data.

Disclosure control

Data associated with destinations in England, Wales and Northern Ireland have been modified 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 (including the commissioned tables). 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. This also applies to data at different spatial levels for the commissioned tables.

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