Blackjack – Avoid The Full-kelly Roller Coaster
June 16th, 2009
Do not be lured by the "magic" of Kelly betting. Although, mathematically, full-Kelly betting appears to be the fastest way to increase a bankroll with no possibility of going broke, if you do not have a replenishable bankroll, full-Kelly betting causes HUGE fluctuations in the process of winning at the "fastest" rate. These fluctuations are absolutely intolerable in real-world games. And they're not tolerable for practical reasons, whether you think you have the stomach for them or not.
Unless you have a very large bank, a full-Kelly betting strategy will often dip your bankroll down to the brink of extinction. If you run a full-Kelly betting pattern on a computer simulation, no problem—your computer couldn't care less if your $10,000 bankroll goes down to $700 before turning around and climbing back up. Nor does your computer mind when your high bets go from $200 to $20, and then back up to $600. But for real-world players, full-Kelly betting is a bankroll roller coaster you'd rather not ride. It's the betting system that performs best on paper, not in real-world casinos.
For a player trying to make it as a pro on a small bankroll (say $10,000), full-Kelly betting is suicide. Players like the thought that with Kelly betting, "it's impossible to go broke." It's true that you can't lose your whole bank if you never bet more than a small fraction of it. But how can you continue playing when your high bets have to be reduced from $100 to $15 because your bankroll took a negative swing from $10,000 to $1500? Where are you going to find a casino that you can beat with $15 high bets?
Negative flux on full-Kelly betting has been the death of more would-be blackjack pros, and has spelled doom for more blackjack teams, than anything else. Talk to some of the old-timers who were playing back in the '70s and '80s when everyone was Kelly crazy. It was a common occurrence for a team to go broke, with one and all chanting: "But we were Kelly betting!" Betting full-Kelly, your winning streaks are way bigger, but your losing streaks are way grimmer. All of the successful big teams today have their players betting very small fractions of the "ideal" Kelly bets.
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