The sample survey is based on information held by HMRC tax offices on persons who could be liable to UK tax. It is carried out annually and covers the income assessable for tax in each tax year.

Samples were selected from three HMRC operational IT systems, which are as follows:
COP:this covers all employees and occupational pension recipients with a PAYE record;
CESA: this covers the self assessment (SA) population; those with self employment, rent or untaxed investment income, directors, those subject to higher rate tax and other people with complex tax affairs. Some people have both a COP and CESA record.
Claims: this covers people without COP or CESA records who have had too much tax deducted at source and claim repayment.

More Source information to be found here ...