Sources and calculation methodology
We prefer primary official sources for government, tax and reference data. A source record distinguishes the publisher, URL, purpose, observation time, source date when available, and freshness limit. A working URL does not prove every statement remains current. Official and market prices are separate data types. Pure arithmetic tools publish their formulas and use deterministic tests; examples are original calculations rather than market forecasts. Failed providers do not create replacement facts. Old observations can remain in history, but expired live values are hidden. Public source links do not imply endorsement.