Let's say he's just trying to say that the vast majority lose. This isn't too hard to believe. From observing the comments/questions/bragging/mistakes I have seen on here over 5 years, it is clear to me that at least 6 out of 10 will blow out within 6 months, because they have no clue whatsover what they're doing - no plan, no method, no discipline, no starting stake.
Of the 4 remaining, 2 or 3 will lack either the capitalization or the will power to stick with it over a number of years until they are profitable. At least 1 or 2 of these 4 will be the type who thought trading was a quick path to riches. They may get to breakeven but will experience a huge drawdown and quit.
There's another thread in here where a guy is asking how long to profitability. Some actual daytraders have responded in there and their average seem to be about 1-2 years just to start breaking even. I've been position trading off of weekly/daily charts for quite a while, but I'm now sticking my foot in the ES intraday, and I'm keeping these figures in mind (and by the way, I think that of 100 guys who start trading the ES with less than 2K, maybe 1 guy will still be standing a year later).
So it is easy for me to believe that 80% are doomed to failure before they their first trade on. Of the 2 guys left, is it so unreasonable to expect that 1 of them will still be in the markets and making consistent profits three years later?
So call it 90%.
Damn lies and statistics....
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EDIT - It is 100% true that those who say it can't be done are the guys who tried and failed.