One System, One Decision, Opposite Results: Why Position Size Decides

Many traders believe that results come from the quality of the signals: find a better system and the returns follow. A controlled experiment at Uppsala University in Sweden showed why that belief is incomplete.

The experiment

A group of 62 participants traded the exact same automated system:

• Same assets
• Same entries and exits
• Same timing
• Same signals

No interpretation was allowed. Each participant started with the same capital and executed roughly 50 trades. They were allowed to make only one decision: position size.

The results

The results were extreme. One participant lost almost all the capital. Another turned the same initial amount into a very large gain. Same system, same market, same signals.

The difference was risk per trade. Participants who failed risked, on average, around 22% to 24% of their capital per position. Participants who succeeded risked between 3% and 7% per trade. Nothing else changed.

Those risking large positions experienced drawdowns they could not recover from. Those keeping positions small survived the volatility, stayed in the game and allowed compounding to work.

Why large positions cannot recover

The arithmetic of drawdowns is not symmetric. A 20% loss needs a 25% gain to recover. A 50% loss needs 100%. A 75% loss needs 300%. A trader risking a quarter of the account per trade can reach the third of those with a short run of losing trades, which any system produces from time to time. At that point the signals are irrelevant, because the capital to act on them is gone.

A trader risking a few percent per trade survives the same run with most of the account intact, and the system's edge, if it has one, gets the number of trades it needs to show up. How to set that percentage and translate it into shares is covered in position sizing strategy, and how drawdown tolerance shapes the whole process in risk management and drawdown tolerance.

Position sizing is not a secondary rule

This is why position sizing sits at the centre of the process rather than at the end of it. A trading system without proper risk control is not incomplete. It is structurally fragile: the same signals can produce a ruined account or a compounding one, and the only variable that decides which is the size of each position.