Thanks for your kind words on this. To clarify however, the Kelly Formulas (as derived, I believe, by Thorp) are applicable to binary outcomes, and satisfy the Kelly Criterion (when the most you can lose is what you put up), but the Kelly Criterion itself is applicable to where there is two or more outcomes.
The failing of the Kelly Criterion is that it uses “returns,” and returns are, de facto, relative to something, absent which, we have nothing but an abstraction. Thus, we see the Kelly Criterion bandied about in the gaming community, where you wager an amount and it is the (maximum) amount you lose. (Yet, in many gaming situations, you can lose more than you put up, blackjack being a prime example with certain rules – so even the gaming community mistakenly applied the Kelly Criterion in the state where it as a mere abstraction).
The problem I faced, in trading, is that what you can lose is not ust what you put up (margin, or, say, the price of a stock when you bought it). Instead, we have short sales, spread trades, forex trades, there are derivatives (e.g. volatility) whose zero-bound is not ever experienced. The Kelly Criterion is not directly applicable to trading (and there are other situations, where, if we play devils advocate and seek to diminish geometric growth, such as reduction of aggregate federal debt) where the abstraction had to be brought to life. Optimal f seeks to do this by scaling the possible outcomes, thus making the "returns"used as input to the Kelly Criterion relative to something. In truth, I would stay away from Kelly’s original formula, in too many situations it will lead you into trouble, and always scale, using the Optimal f formula to, in effect, solve for the Kelly Criterion.
Here’s the other problem – solving for the Kelly Criterion gives you the asymptotic growth optimal fraction to risk. That is, it yields the growth optimal fraction only as the number of trades or periods approach infinity.
By way of an example, suppose you have a system that will profit 51% of the time, and lose the other 49% of the time. Let’s keep it very simple and say that what is the amount you expect to profit by is inverse of the same amount you would expect to lose by. In our case, we are only going to be allowed to make one trade, then we can never trade again.
We calculate the expected growth optimal fraction in this case to be 2%. However, that 2% is if we were to make ever-repeated plays under these possible outcomes. In actuality, we maximize our expected geometric growth if we quit a positive expectation game at one play by risking 100% of our stake. If we quite at 2 plays, the expected growth optimal fraction will be at somewhere between 100% and 2%, and will keep diminishing approaching 2% as the number of plays approaches infinity.
So the Kelly Criterion solution is never the expected growth optimal solution, but rather only its asymptotic value, and absent being made relative to something, it is a mere abstraction at that and one that can lead you into lots of over-exposure situations.
The math for all of this is on my website, which is my name dot com, and on the “Optimal f” tab at the top. I’m not trying to hustle anything here, just trying to contribute to the conversation with like-minded guys. Thanks, Ralph Vince