How a number earns its place
Step 01
Source
A price must appear on the provider's own public page. We record the URL and a verbatim quote of the sentence that states it.
Step 02
Date
Every observation carries the day we verified it. The “checked” stamp on a card is that date, not a decoration.
Step 03
History
Observations are append-only. A price change adds a new dated point; the old one is never rewritten. That is what the charts plot.
Step 04
Sanity gate
A price that moves more than 40% in one check is held for human review before it publishes. Extraction errors look exactly like price drops.