All policy costings presented to the Office for Budget Responsibility at Budget 2018 were scrutinised and were certified as reasonable, central estimates were included in our forecasts. The Government’s Budget 2020 policy costings document briefly describes the methodologies underpinning these costings. In our March 2020 Economic and fiscal outlook we have published an assessment on the level of uncertainty associated with each of the certified costings. For those identified as being particularly uncertain, we provided greater detail on the most important sources of uncertainty.
Read more in Annex A of the Economic and fiscal outlook