Where the postcode is not supplied, there is a clear bias towards larger non-residential transactions, and as these cannot be allocated to a local area, the resulting statistics are not reliable.
Lle na chaiff y cod post ei nodi, mae gogwydd amlwg tuag at drafodiadau amhreswyl mwy o faint, a chan na ellir eu dosbarthu i ardaloedd lleol, nid yw'r ystadegau canlyniadol yn ddibynadwy.