Table C0723 was created as commissioned output from the 2001 Census. It disaggregates migrants by age and ethnic group. For ease of use it has been split into two parts:

• C0723A - Flows from regions to wards
• C0723B - Flows from wards to regions

The original data are asymmetric: that is, the origin geographies and destination geographies are not the same. The data set was designed in this manner in order to reduce the impact of Small Cell Adjustment Methodology (see section on disclosure control below). By having a coarse origin geography in C0723A, flows into individual wards may be large enough to be unaffected by SCAM. Likewise, outflows from wards may be large enough in C0723B to be unaffected.

Spatial scope

The data set was prepared by ONS, and includes those persons resident in England and Wales. Thus, migrant destinations in C0723B are limited to those in England and Wales. Migrant origins include all areas of the UK, and counts of migrants from outside the UK are also included.

The origins used in C0723B are Standard Table wards in England and Wales, whilst the destinations used are Government Office Regions. In addition, Scotland and Northern Ireland are included as separate origins.

The data as originally supplied include counts for total flows from Scotland and from Northern Ireland to GOR areas in England and Wales. The data set as made available by CIDER also includes total from each GOR in England and Wales to itself and all other GORs in England and Wales. It is important to note that these totals were summed from the original ward-to-GOR values all of which had been subject to Small Cell Adjustment (see below). The flows from Scotland and Northern Ireland, however, were only subject to SCAM after aggregation.

Disclosure control

All data in this set 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.