The decision journal, and why almost nobody keeps one

Ask someone what they expected before a decision that went badly, and you will get a reconstruction. Not a lie: a genuine memory, revised by knowing how it turned out. This is one of the more robust findings in judgement research, and it makes learning from experience much harder than it appears. You cannot calibrate against a record that edits itself.

What an entry needs

• The decision, in one sentence someone else could act on.
• The options you actually weighed, including the ones you rejected quickly.
• The assumption it rests on. Usually there is one load-bearing belief, and naming it is most of the value.
• Your confidence, as a number. Vague confidence cannot be checked later.
• A review date, and what you would expect to see by then if the assumption held.

The assumption is the entry. Everything else is context.

Why it does not stick

Because the cost is immediate and the payoff is months away. Ten minutes now, value in a quarter, and no feedback in between. That is close to the worst possible incentive structure for a voluntary habit, which is why decision journals are widely admired and rarely maintained.

The fix is not discipline. It is to make the record a by-product of something you were doing anyway. If working the decision produces the entry automatically, and the review date comes back to find you, the habit no longer depends on willpower.

What it gives you after a year

Patterns you cannot see from inside individual decisions: the kind of bet you keep making, the assumption you keep being wrong about, whether your confidence is calibrated at all. Most people discover their confidence is not well calibrated, and that finding alone is usually worth the year.

Five Peers editorial note. Sources named in the text.