Why position size matters more than stock selection

Ask somebody why their portfolio disappointed and they will name a stock. It is almost never the stock. It is that they held four times as much of it as of anything else, because they felt most certain about it — and certainty and correctness are not the same variable.

The uncomfortable arithmetic

If you hold twelve names equally weighted, a name that falls 50% costs you about 4% of the portfolio. If that same name is a quarter of the portfolio because you liked it most, it costs you 12.5%. The stock behaved identically in both cases. Your outcome differed by a factor of three, and the only variable you controlled was size.

Fix the size before you read the idea

This is the practical rule and it is deliberately awkward: decide what a position is worth before you have read the argument for it. Reading a good argument first and then sizing means the size is set by how persuasive the writing was, which is a measure of the writer, not of the risk.

Three consequences worth accepting

• Your best idea will be the same size as your fourth-best. That is the cost of not knowing in advance which is which, and it is a real cost, paid in exchange for surviving the times you were confidently wrong.
• You will run out of room. Good. Full compartments are a reason to stop buying, not a reason to size the next one smaller and pretend the rule held.
• A predictable share of ideas will not work. Not a sign of failure. A research process where every idea worked would be a research process with far too few ideas.

The position size calculator does the arithmetic, and the Money Management Rules set out the whole framework.

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Why this explainer exists. It is the whole basis of the Money Management Rules, which are published free and outside the paywall.