Investing term
What is Decision journal?
A pre-trade written record of your thesis, sizing, and what would change your mind.
A decision journal is a written record made before you trade, capturing your reasoning, how big the position is, and what would prove you wrong. You write it in the moment of deciding — the thesis, the catalyst, the risks, the invalidation, the size — so there's an honest record of what you actually thought.
Its power comes later. When you review it after the outcome, you see your genuine thinking rather than the rewritten memory your mind invents once you know how things turned out. Hindsight quietly edits the past — 'I always knew that was risky' — and the journal is the antidote, preserving what you really believed. It lets you separate good decisions from lucky ones, and bad decisions from unlucky ones, which is the only honest way to learn from your own investing.
A decision journal records your thesis, size, and invalidation before you trade. Reviewed later, it shows what you really thought — not the memory hindsight rewrites — so you learn from process, not luck.
For example
Before buying, you write: 'Thesis, target, what would make me sell, 4% position.' A year later you compare that record to what actually happened — and to what you now remember thinking.
Learn it by doing
That's Decision journal in theory — it clicks when you use it. Practise it hands-on in a free, interactive lesson (Stage 13, Active Investing: Should You Even Bother?).
Try the free lesson →Why it matters to you
A decision journal matters because it's the single best tool for learning honestly from your own decisions. Without one, hindsight bias rewrites your memory to match outcomes, so you draw the wrong lessons — crediting luck as skill and blaming skill as bad luck. The journal freezes your real reasoning at the moment of decision, letting you judge the process rather than just the result. Over time, that's how you actually improve as an investor.
⚠ Judging decisions by outcomes alone
Without a written record, you'll judge past decisions by how they turned out — calling a lucky win 'skill' and an unlucky loss 'a mistake'. But a good decision can have a bad outcome and vice versa. A decision journal preserves your actual reasoning so you can evaluate the process, not just the result, which is the only reliable way to improve.