How to Build a Decision Journal (and Why Your Future Self Needs One)

A decision journal is evidence against your future self

The purpose is not to write a diary. It is to freeze the reasoning at the moment when the decision is made, before the share price tells you whether to feel clever or foolish.

A stock that rises can make weak reasoning look brilliant. A stock that falls can make a sound, probabilistic decision look stupid. If you only grade the outcome, you learn the wrong lesson.

What I would record at entry

Keep the original entry short enough that you will actually maintain it. Record the date and price, the investment thesis in two or three sentences, the key facts supporting it, the valuation or expectations embedded in the price, and the position size.

Then add the most important line: what would make the thesis wrong? Not 'the stock falls 20%'. Write the business evidence that would invalidate the idea - margin deterioration, loss of a major customer, leverage moving beyond a threshold, a product failing to gain adoption, or whatever matters for that company.

Separate facts, forecasts and feelings

One of the best improvements is to label each statement. 'Revenue grew 12% last year' is a fact. 'Revenue can grow 10% for the next five years' is a forecast. 'I think management is excellent' is a judgement.

When these are mixed together, conviction becomes difficult to audit. When they are separated, you can see later whether the loss came from bad data, a bad forecast or a reasonable thesis that simply encountered an adverse outcome.

Never overwrite the original thesis

When new information arrives, add a timestamped update. Do not edit the old entry until it looks like you always knew what happened.

This is crucial. The value of a journal comes from preserving the gap between what you believed then and what you know now.

Review on triggers, not only when you sell

A journal should resurface when something meaningful changes: earnings, guidance, a material acquisition, a balance-sheet deterioration, a valuation extreme or a major technical breakdown if trend is part of your process.

At each review, ask: Which original assumptions still hold? Which have changed? Has the position size drifted? Is the expected return still competitive with alternatives?

Grade the process

When the position closes, score the decision on reasoning and execution before looking at the P&L verdict. Was the thesis testable? Did you size it consistently with uncertainty? Did you react to disconfirming evidence or rationalise it away? Did you follow the exit rule you wrote down?

Over enough decisions, patterns become visible. Maybe you systematically overestimate turnarounds. Maybe you sell compounders too early. Maybe you ignore valuation when a narrative is exciting. That is where a journal earns its keep: it converts vague self-awareness into a specific process you can improve.