The sample is selected from people who previously took part in the face-to-face version of the National Survey and agreed to take part in further research.
Caiff y sampl ei ddewis o blith pobl sydd wedi cymryd rhan o’r blaen yn y fersiwn wyneb yn wyneb o’r Arolwg Cenedlaethol ac a gytunodd i gymryd rhan mewn ymchwil bellach.