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.
Ystyrir bob amser opsiynau o ran a fyddai’n fwy cost-effeithiol defnyddio data sy’n bodoli eisoes, diwygio arolwg sy’n bodoli eisoes neu gynnal arolwg newydd.