Externalize your assumptions

The idea

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.

In 1993, James Dewar and his colleagues at the RAND Corporation published a methodology that changed how the U.S. Army planned for an uncertain future. The insight behind Assumption-Based Planning was not that plans fail — everyone knows that. The insight was why they fail: every plan rests on assumptions, and the assumptions most likely to be wrong are the ones nobody wrote down.

An example

A startup spends eighteen months building a product around the assumption that enterprise buyers will pay for a self-serve onboarding flow. Nobody writes down this assumption. Nobody tests it. When they finally talk to buyers, they discover enterprises require white-glove implementation — not because self-serve is bad, but because procurement processes mandate human touchpoints. The assumption was never wrong in theory. It was wrong in context. And because it was never externalized, it was never examined until $2M was gone.

Where it goes wrong

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 have not gone deep enough. The real practice is surfacing what you do not know you are assuming.

Make it stick

Before your next significant decision or project kickoff, create an assumption register — a written list of every premise the plan depends on. For each assumption, note what would change if it were false. Review the register weekly. When an assumption is validated or invalidated, update the register and adjust the plan. This is not overhead. This is how plans survive contact with reality.