18 The SAIL Databank has been previously used for linkage of routine data.22–26 Within the SAIL Databank a split-file approach to anonymisation is used to overcome issues of confidentiality and disclosure in health-related data warehousing.18 Demographic data are sent to a partner organisation, NHS Wales Informatics Service (NWIS), where all identifiable information is removed;

clinical this website data are sent directly to HIRU, where, for each data set within the SAIL Databank, an individual is assigned an encrypted Anonymised Linking Field (ALF). The ALF is used to link anonymised individuals across data sets, thus supporting the opportunity to conduct longitudinal analyses of an individual’s journey through multiple health, education and social data sets.17 Additionally, Residential ALFs (RALFs) have been created for all residences in Wales and enables linkage of anonymised household and environment data with the health records of individual residents without the identity

of the residences or residents being known to researchers.26 The primary study base will be the Welsh Demographic Service27 (WDS) hosted within the SAIL Databank. The WDS is a core data set available within the SAIL Databank and part of a set of services to manage administrative information (demographic data) for NHS patients in Wales. The WDS was introduced early in 2009 replacing a similar service known as the NHS Wales Administrative Register (NHS AR). The WDS data is collected from GP’s via the Exeter System; more than five million individuals are currently present in the WDS data set within the SAIL Databank. The WDS is a register of all individuals who have at some point in time been registered with

a Welsh GP or required some form of NHS healthcare provision in Wales. The electronic collation of WDS/NHS AR data originated in 1960, and is updated and maintained by NWIS,27 ensuring that address changes (within and out of Wales) and death notices are included in the register. The original (non-anonymised) version of the NHS AR has been used in the HIRU Matching Algorithm for Consistent Results in Anonymised Linkage (MACRAL), making the WDS/NHS AR the master list for all Welsh residents and using probabilistic matching to find the associated GSK-3 NHS numbers that are then encrypted into ALF’s. Deaths in Wales should be registered within 5 days of the date of death (DOD). However, legislation in Wales means that when a coroner’s inquest takes place, the death cannot be registered until the inquest is complete. Since the Office for National Statistics (ONS), the national agency where all deaths are collated, has no conclusive information about the death until it is registered, there is a delay between the date the death occurred and when the death is added to the annual ONS mortality data set.