Comparisons over time are possible, for topics that were included in previous years of the National Survey (i.e. as far back as 2012-13).
Gellir cymharu canlyniadau dros gyfnod o amser ar gyfer pynciau a oedd wedi'u cynnwys mewn Arolygon Cenedlaethol blaenorol (hynny yw, yn ôl i 2012-13).