Just to clarify the 90/10% rule.
This does not imply 90% of the trades are losers or the traders have a unusually low win rate, in his studies the win rates were not a factor. The 10% are the very large winners that really move the equity curve.
When done well, the 90% pile is a big scratch out. The less profitable winners pay for the losers, these are trades that hardly move the equity curve up or down.
Ken Grant analyzed the trades from a large sample of portfolio managers (real professionals, like Steven Cohens traders) and found that for nearly every account, the top 10% of all transactions ranked by profitability accounted for 100% of the P/L for the account. In many cases, the 100% threshold was crossed at 5% or lower.
The legendary day trader Pit Bull (Marty Schwartz) in Market Wizards, maybe the best day trade of all times, said this:
"For two hundred days a year, I’d end up with reasonably small losses netted out with similar-sized gains. Lose $5,000 here, make $6,000 there, round after round, twenty, thirty, forty times a day. But I’d win the other fifty trading days by clear-cut unanimous decisions." Marty Schwartz