This resulted in a reduced sample size and the characteristics of respondents was different to previous years which had implications for the quality of the data and how it was presented.
O ganlyniad, cafwyd sampl llai o faint ac roedd nodweddion yr ymatebwyr yn wahanol i flynyddoedd blaenorol a effeithiodd ar ansawdd y data a'r modd y cawsant eu cyflwyno.