What is an Assumption Log?
The assumption log records what you're treating as true without proof (assumptions) and what you must work within (constraints), from day one onward. Every estimate and plan is built on assumptions — writing them down converts invisible bets into visible, testable ones.
The discipline is revisiting it: assumptions get validated or busted as facts arrive, and a busted assumption is a risk being born. "We assumed the client would provide test data by June" is either confirmed — or it's next month's schedule slip, spotted early.
Worked example
A solar-farm project logs: "grid connection approved within 90 days" (assumption, historic average), "site access limited to daylight hours" (constraint), "panel prices stable through Q3" (assumption, market-based). When panel prices spike in July, the team isn't blindsided — the log flagged which bet just failed and what it touches.