Options are always considered as to whether it is more cost effective to make use of existing data, to amend an existing survey or to carry out a new survey.
Mae opsiynau yn cael eu hystyried bob amser i weld os yw’n fwy cost-effeithiol i wneud defnydd o'r data sy'n bodoli eisoes, i ddiwygio arolwg presennol neu i gynnal arolwg newydd.