The sample frame is limited to reporting units that have local units in Wales and have existed long enough to be able to give the full year of data being asked for.
Mae ffrâm y sampl wedi'i chyfyngu i unedau adrodd sydd ag unedau lleol yng Nghymru ac sydd wedi bodoli'n ddigon hir i allu rhoi'r flwyddyn lawn o ddata y gofynnir amdani.