rob carver ran a multi-billion dollar portfolio at Man AHL and has written five books on systematic trading. his rule for anyone who wants to use discretion: you get one decision. "you have to limit the discretionary part of a trading strategy to one tiny part, which is the what should I buy and when decision. every other part should be based on a scientific, systematic, rule-based approach. how fast you trade, when to close positions, and how big positions should be." "if taking poor buy decisions was the only mistake retail traders made, you'd expect roughly half to lose money and half to make money, because worst case they're flipping a coin. but the real number is more like 95% of retail traders lose money." "that's because they're making errors with position and cost and risk management. and that's where there is no room for discretion. there's no art involved in that at all. that is science." entries are the part most people argue about. but sizing, costs and exits are where the 95% actually happens.