The answer I gave the hon. Member for Shipley (Philip Davies) on 16 December 2009, Official Report, columns 1246-47W, presented figures on offences committed by offenders while on Home Detention Curfew (HDC). These figures were obtained by matching a dataset of offenders released on HDC with offending data taken from an extract of the police national computer (PNC). There is no unique offender ID common to the two datasets. The matching process uses each offender's surname, forename, gender and date of birth and involves both direct matching and a variety of ‘sounds like' algorithms to allow for minor errors in data entry. A small proportion of cases in each quarter cannot be matched; this can result from errors in the recording of personal details in either of the data sources, from duplicate matches when it is not possible to decide between PNC records for offenders with similar details, or from records missing from the PNC. In 2007-08 one per cent. of the offenders listed on the prison data as released on HDC could not be matched to the PNC.