Position Sizing Is the Strategy. The Stock Pick Is a Detail.

There is a thought experiment worth running. Two investors identify the same ten businesses over a decade. They buy and sell at the same times. One ends the decade with a materially better result than the other. Nothing about their stock selection differed — only how much they put into each name.

Why sizing dominates

The arithmetic is unforgiving. A position that is 2% of a portfolio can triple and add six percentage points. A position that is 20% can halve and remove ten. In a portfolio of ten to twenty names, the distribution of position sizes has more influence on the outcome than the distribution of returns, because returns cluster more tightly than most people expect while sizing decisions do not.

This is uncomfortable because sizing is the part of investing that receives the least attention. Idea generation is interesting. Sizing is arithmetic and discipline.

What a sizing framework needs

A workable framework answers three questions before capital is committed: how confident are you in this thesis relative to your others, how much of the portfolio can this position lose without forcing a bad decision elsewhere, and how correlated is it with what you already own.

The third question is the one most often skipped. Six positions in different companies that all depend on the same interest-rate path is one position wearing six hats.

A starting structure

• Set a maximum single position size and never exceed it, whatever the conviction.
• Set a maximum sector or theme exposure — this catches the correlation problem.
• Size by conviction bands rather than continuously; three bands is usually enough.
• Decide the size before you decide to buy, so the size is not rationalised by enthusiasm.

The framework matters less than having one. Any consistent rule outperforms sizing by how excited you felt on the day.