Two investors generate the same 60% win rate, identifying winners and losers with identical analytical skill. Investor A sizes every position at 5%; Investor B sizes winners at 2% and losers at 12% by 'averaging down.' Same skill, same names, same time horizon.

Investor A compounds at roughly 12% per year; Investor B compounds at zero — or worse — because the asymmetric sizing converts every wrong call into an oversized portfolio drag. Position sizing decides more of your long-term return than stock selection does. The math is unforgiving: a 75% loss requires a 300% gain to recover, and a 90% loss requires 900%.

This lesson is about how much to allocate to each idea — and why the question is far more important than which idea to pick. Position sizing answers a question most retail investors don't even realize they're answering: how much of my portfolio do I bet on this idea?