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I just made a great trade! I closed at a 10% loss, but don’t mind the noise. I’m celebrating with champagne.

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The Treachery of Treating Single Outcomes As Evidence

There is a temptation so universal, it barely registers as a mistake. You do something; it goes well. You conclude you did the right thing. It goes badly. You conclude the opposite. This seemingly “obvious” process risks passing without scrutiny in many domains of human decision-making: trading, business planning, capacity forecasting, or any such domain with uncertain futures that require irreversible commitments. This article is about the error with this process, and the mathematics that can help you escape mediocrity.

Part I: The Losing Trade

Suppose you execute a trade. You have a thesis: the price of some asset is likely to move in a particular direction over some time horizon, and you have structured a position to profit from that movement. The position closes. You lose money.

The natural response, especially for a new trader or a trader under pressure to justify their book, is to treat this loss as evidence that the thesis was wrong.

But this may be incredibly misleading because the outcome was revealed after the decision to initiate the trade. This gap makes the outcome signal too noisy to use in isolation.

Consider what actually happened. You operated inside a probabilistic environment…