Based on research. Anyone who's interested can start with Odean's work if they like. However, rather than perseverating on some number, the individual who wants to succeed will focus on the reasons for success or failure, which are essentially the same as they were a hundred years ago a la Wyckoff and Livermore: don't overtrade, sell your losers, keep your winners.
As to being "heartening", it shouldn't be. Or disheartening, for that matter. If one truly wants to be a trader, the success or failure of others, regardless of how one defines it, is completely irrelevant.