Why does assumption testing fail?

The most common reason assumption testing fails: Listing only the assumptions you are already aware of — the safe, obvious ones. The assumptions that destroy plans are the ones so deeply embedded you mistake them for facts. If your assumption list feels comfortable, you haven't gone deep enough. The real practice is surfacing what you don't know you're assuming.

The fix: Pick one active project or decision. Set a timer for ten minutes. Write down every assumption you can identify — about the people involved, the timeline, the resources, the market, the technology, your own capabilities. Aim for at least fifteen. Then mark each one: (K) for assumptions you have evidence for, (U) for assumptions you are uncertain about, and (L) for assumptions that would be catastrophic if wrong. Any assumption marked both U and L is your highest-priority assumption to test this week.

The underlying principle is straightforward: Assumptions you never write down are assumptions you never question. Every plan, decision, and belief rests on invisible premises — and the invisible ones are the ones that destroy you.