Every plan rests on things being true that nobody has checked. Not risks — those get written down. Assumptions: the quiet load-bearing beliefs that felt too obvious to state. A pre-mortem is the exercise that drags them into the open, and it does so by a trick of framing that turns out to matter enormously.
What a pre-mortem does
The method was popularised by the psychologist Gary Klein. Before work begins, the team is told to assume it is a year in the future and the initiative has failed — not might fail, has failed. Their job is to write the story of how. The failure is presented as a fact, and the group is asked only to explain it.
The output is not a list of risks. It is a narrative, and the assumptions it exposes are usually ones no risk register contained, because a risk register asks what might go wrong and an assumption is a thing you did not think could.
Why it works when a risk review does not
The difference is social, not analytical. In a normal review, raising a serious doubt means arguing that a plan your colleagues have worked on is bad. That is expensive, and people who have read the room stay quiet. Reasonable objections go unsaid, and everyone leaves believing the silence was agreement.
A pre-mortem removes the cost. Nobody is arguing the plan will fail; the premise says it did. Explaining a failure that has already happened is an invitation to be insightful rather than difficult. The same person who would not have objected will now describe, in detail, the exact thing they were worried about.
The practical consequence is that a pre-mortem gets you information that exists in the room and would otherwise have stayed there.
The four-step method
One: state what success is, precisely enough to be wrong. Not "the launch goes well" but "forty active partners and $250k of sourced revenue by the end of Q3". A vague target cannot fail, which means it cannot be pre-mortemed.
Two: list what must be true. Work backwards from success. For each step, ask what has to hold for it to happen. Keep going until the list contains things that feel obvious — those are usually the load-bearing ones.
Three: score each assumption twice. How confident are you that it holds, as a percentage? And how much does the plan depend on it, on a scale where five means the plan is over if this is false? Two numbers, because they are different questions and the dangerous assumptions score badly on both.
Four: write the failure. Take the assumptions that are heavily depended on and weakly believed, and write the post-mortem as if it were a year from now. Name what happened, in plain language, in the past tense.
The arithmetic nobody does
Here is where most pre-mortems stop short, and it is the part that changes minds.
Ask a team how confident they are in each assumption and you will get a series of reassuring answers: eighty percent, eighty-five, ninety. Every one sounds like a plan in good shape. But the plan does not need one of them to be true. It needs all of them to be true, at the same time.
Six independent assumptions at eighty percent each give you 0.86, which is twenty-six percent. A plan everyone in the room would describe as solid has roughly a one-in-four chance of surviving contact with its own premises. Nobody multiplied, so nobody saw it.
This is not a trick of arithmetic; it is the arithmetic. And the honest version is worse. Multiplying assumes the assumptions are independent, and real ones rarely are — the optimism that inflated one usually inflated its neighbour, and a single slipped quarter can break three at once. Correlated assumptions fail together, so the true odds sit below the multiplied figure rather than above it. Treat the number as the optimistic reading and act accordingly.
The useful move is not despair. It is to notice that the multiplication tells you exactly where to spend effort: the assumption dragging the product down hardest is the one to go and test this week.
A worked example
A team plans a partner marketplace. Success is forty active partners and $250k of sourced revenue within twelve months. Working backwards, four things have to be true:
- Partners will integrate without engineering support from us — 55% confident, and the plan depends on it completely.
- We can sign forty partners in the window — 65% confident, plan depends on it completely.
- Existing customers want a marketplace at all — 75% confident, heavily depended on.
- Legal can clear revenue-share terms in one quarter — 80% confident, materially depended on.
Every one of those is a defensible answer. Multiply them: 0.55 × 0.65 × 0.75 × 0.80 = 0.21. A twenty-one percent chance the plan survives its own assumptions, and the team had not noticed, because they had never been asked the question in a form that required multiplying.
The ranking also tells them what to do first. The self-integration assumption is both the least believed and the most depended on, so it is the most fragile thing in the plan. That is a week of work with three prospective partners, not a quarter of building.
Tripwires
The last step is the one most often skipped, and it is what turns a pre-mortem from an interesting afternoon into a control.
For every load-bearing assumption, write down what would show early that it is turning out false, and by when. "Fewer than five partners past technical review by week six." "Legal has not returned first comments by the end of month two." Specific, observable, dated.
An assumption you cannot check is a hope. A tripwire converts it into something the plan can respond to while responding is still cheap — which is the entire practical value of having done the exercise at all.
What a pre-mortem is not
It is not a forecast. The percentage is a structured expression of the team's own beliefs, not a measurement of the world, and it should be presented that way to a board.
It is not a veto. A twenty-one percent plan may still be the right plan if the payoff is large and the cost of failing is small. What the number changes is how you resource it, what you test first, and what you tell your sponsor.
And it is not a substitute for a risk register. The two answer different questions: a register tracks known threats through delivery, a pre-mortem interrogates the premises before delivery starts. Run the pre-mortem first; it will usually put several new entries into the register.
Frequently asked questions
What is a pre-mortem?
A pre-mortem is a planning exercise run before work starts. The team imagines it is a year in the future and the initiative has failed, then writes the story of how. Because it asks people to explain a failure rather than predict one, it surfaces doubts that a normal risk review leaves unsaid: nobody has to argue the plan is bad, only to explain a failure that is presented as already having happened.
How is a pre-mortem different from a risk register?
A risk register lists things that might go wrong and scores them. A pre-mortem starts from the assumptions the plan needs to be true and asks what happens when they are not. The difference matters because the assumptions most likely to sink a plan are usually the ones nobody wrote down as risks, precisely because everyone took them for granted.
Why do confident plans fail so often?
Because confidence does not compound the way people expect. A plan that needs six independent things to be true, each of which you rate at 80 percent, has a 26 percent chance that all six hold. Every individual answer sounds reassuring and the joint answer is not. Almost nobody multiplies, so almost nobody sees it.
Does multiplying the confidences overstate the risk?
It would if the assumptions were negatively correlated, and that is rare. In practice assumptions correlate positively: the same optimism that inflates one usually inflates its neighbour, and the same slipped quarter breaks several at once. Correlated assumptions fail together, which makes the real odds worse than the product, not better. Treat the multiplied figure as the optimistic reading.
What should a pre-mortem produce?
Three things. A ranked list of which assumptions are most fragile, meaning heavily depended upon and weakly believed. A written narrative of the failure, in plain language, dated in the future. And a tripwire for each load-bearing assumption: what would show early that it is turning out false, and by when. An assumption you cannot check is a hope.