Forecasting – The Comparable model will use forecasts of UK government income tax up to the autumn prior to the start of each financial year in question.
Rhagolygon – Bydd y model Cymaradwy yn defnyddio rhagolygon o dreth dirlenwi llywodraeth y DU hyd at yr hydref cyn dechrau pob blwyddyn ariannol benodol.