The WDS provides a history of residences where a person has registered as living, updated every time they sign up to a General Practitioner (GP) in Wales or change their address with their GP.
Un o'r ffynonellau data a archwiliwyd gan y SYG oedd Cofrestr Gleifion y GIG, a oedd yn cynnwys cofnod ar gyfer pob person a gofrestrwyd gyda Meddyg Teulu GIG yng Nghymru a Lloegr.