The Comparative Analysis Service receives data from local schemes; these data are to be extracted from all the practices within each participating scheme. The first data extraction will take place in January 1998, and will be used primarily for the purpose of providing baseline data upon which progress in data quality improvement over the period of the pilot stage can be measured. The data will also be analysed to produce a range of routine reports including comparative results at various levels (including comparisons with local and national demographic data), and ad hoc reports at the request of users. In producing reports that compare data from a range of practices and between different health authorities, suitable adjustments may need to be made for different population profiles, for differences in recording methods, and other factors. There will be considerable reliance on local Public Health knowledge and interpretation, supplemented by advice from the Department of Public Health Medicine and Epidemiology at the University of Nottingham.

The Comparative Analysis Service will work only with data collected by local schemes from which all patient and practice identifiers have been removed. The core data set as described in the CHDGP Guidelines (or selected parts of it, by agreement) will be provided by all contributors to the Comparative Analysis Service by means of standard queries prepared by the Service. In addition, some schemes will (by agreement) regularly provide additional data for inclusion in comparative analyses.