Reliable evidence is required throughout policy development in planning, during implementation and in evaluating whether the policy was successful and where improvements are required.
Mae angen tystiolaeth ddibynadwy drwy gydol y broses o ddatblygu polisi er mwyn cynllunio'r polisi, rhoi'r polisi ar waith a gwerthuso a oedd y polisi'n llwyddiannus a beth sydd angen ei wella.