Details of the data collection and validation processes, strengths and limitations of the data, and revisions policies can be seen in the quality and methodology sections of the original publications:
Gellir gweld manylion y prosesau casglu a dilysu data, cryfderau a chyfyngiadau'r data a pholisïau adolygu yn adrannau ansawdd a methodoleg y cyhoeddiadau gwreiddiol: