In a study that used data drawn from the first 6 waves of the survey 2009 to 2010, 2015 to 2016, Boyd et al. (
Mewn astudiaeth a ddefnyddiodd data o chwe chylch cyntaf yr arolwg, 2009 i 2010, 2015 i 2016, rhestrodd Boyd et al. (