The Budget number that can move
Every costing comes with an uncertainty rating. Read that before the speeches.
- The claim
- "This tax change will raise a fixed amount of money."
- Why it doesn't add up
- Every official costing is a central estimate, and the watchdog that checks it gives each one an uncertainty rating, from low to very high, because people change what they do when a tax changes. The headline figure is the middle of a range, not a promise.
- How it could be better
- Report the watchdog's uncertainty rating next to the headline figure, every time.
Explore the detail
The OBR gives every certified costing a subjective uncertainty rating from "low" to "very high" based on data, modelling and behavioural response (OBR, "Consideration of uncertainty in OBR fiscal forecasts"; policy costings database). Costings are "highly sensitive to assumptions about the future behavioural responses of taxpayers" (OBR Briefing Paper No. 6, para 3.23). The headline is a "reasonable and central estimate" with risks on both sides (OBR EFO Annex A, para A.21). The Budget is on Wednesday 28 October 2026 (gov.uk, Chancellor's letter to the Treasury Committee). All VERIFIED.
Last checked 9 Oct 2026. Corrections: Corrections policy · Archive