Thank you sir. Those 2 books are winging their way to me as we speak (along with several CDs...damn you Amazon!)Originally Posted by Ganchrow
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To some extent ... as the market adjusts itself the predictive strength of any given forecast will, in general, decay over time. This will tend to manifest itself in one of two ways.Originally Posted by trustbutverify
Firstly, there might be a decrease in the number and strength of forecasts that meet your hurdle rate.
Secondly, and potentially more harmful to a market participant, the forecasting power of the model might decrease, resulting in biased forecasts. The problem is that the player would be overestimating his edge on any given bet, meaning that not only would he make less money (as in the "firstly"), but also he'd be unable to optimally manage his risk. But ultimately, it would just be up to the player to create a robust enough model to properly account for this factor and a flexible enough modelling framework to rapidly adjust for changes in regime.
SBR Founder Join Date: 8/28/2005
I'm not sure entirely certain what you're getting at here, but I fear you might be treading dangerously close to the Gambler's Fallacy.Originally Posted by Dark Horse
Just to be totally clear: assuming a 1/75 fail probability, the probability of the shuttle failing at least once at some point over the next 1,000 launches would be (1-1/75)^1000 ≈ 99.99985%.
However, given that the shuttle has not failed at any point in the last 999 launches, the probability of it failing on the thousandth launch would still be 1/75.
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You just lost me.Originally Posted by Dark Horse
Well guess what ... gambling can be a bit of a gamble.Originally Posted by Dark Horse
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Always good to meet someone who has truly embraced risk.
(Time still has a few mysteries.).
SBR Founder Join Date: 12/14/2005
Even if you didn't feel comfortable making the frequent edge approximations required by Kelly and have resigned yourself to assuming an equivalent edge on each bet you placed, that doesn't necessarily mean that traditional flat-betting would be your best option. In fact, it probably won't be.
If lowering the variance of your bankroll's growth were of concern to you (and it certainly should be) and you frequently find yourself betting across a wide variety of money lines, then you might want to consider moving away from fixed unit staking towards a "fixed-profit" staking plan. Fixed-profits staking refers to betting to win a constant amount on all bets. So in other words, if a fixed-profits staker were to bet 1 unit at a line of +100, he would be betting 1.1 units on a money line of -110, and ˝ of a unit on a a money line of +200.
Joseph Buchdahl in Fixed Odds Sports Betting: Statistical Forecasting and Risk Management demonstrates how a bettor engaging in fixed-profits staking can reduce both his standard deviation and his risk-of-ruin versus a flat bettor with the same average bet size.
(Fixed-profits staking, btw, is actually implicit in Kelly betting.)
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I'd say it's often as much naďveté as it is greed. The decision to bet fixed profits on payout odds less 1:1 but fixed stake on payout odds greater than 1:1 might be little more than the product of US-style lines display and an unimaginative mind.Originally Posted by trustbutverify
It generally comes out as Buchdahl's simulation predicts: Roughly equal return and lower standard deviation of finishing bankroll than with fixed profits.Originally Posted by trustbutverify
What can't easily be determined from looking at a single sequence, however, is the fact fixed-profit staking also boasts a higher probability of being profitable over any given stretch, along with a lower risk-of-ruin probability.
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No. If you're betting more on short odds relative to long odds, then your average break-even win percentage would have to increase.Originally Posted by trustbutverify
But all this really proves is that break-even win percentage is not a very meaningful statistic when considering bets of varying odds.
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That's very nice of you to say. Thank you.
To an economist, this is obviously quite clear. I never meant to imply this was solely applicable to Kelly qua Kelly. If you read through much of the forum literature on Kelly then what you'll find are that many to most fairly quantitative posters mistakenly claim that the Kelly bankroll is a subset of total net worth. Obviously these good people aren't economists.
Again, I hadn't mean to imply imply that this behavior was solely applicable to log prefs. Clearly its applicable to a set of preferences of which logarithmic are but one. The important point isn't that given the certainty of an event one would bet all under log utility, but rather that given a sufficiently near certainty, log utility would imply one would bet so much as to risk any specified fate not quite so bad as death (assuming death to be infinitely bad).
The answer is yes it does. Maximizing log utility is functionally equivalent to maximizing expected bankroll growth (or some constant fraction thereof). If a market participant's goal is the latter then that implies his utility function is logarithmic. If a market participant's utility is logarithmic, then that implies he will act to maximize expected bankroll growth. Realize that we can talk theoretically about maximizing expected bankroll growth even if we're only dealing with a single time period.
Kelly only deals with "snapshot" utility. If you wanted to include a notion of maximization over some time horizon you'd have to come up with some estimate of the distribution future betting opportunity. We would expect this to be superior to Kelly.
We're wrapping the log around wealth to cause players to choose to maximize expected bankroll growth. Diminishing marginal utility naturally follows. Again, if we wanted to throw either an integral or a summation around the utility function and add subscripts and some time-value-of-money measure we certainly could do so. Is it worth the effort? Unless you're typically operating near full capacity with trades expiring over some wide swath of time periods, I'd say that from a nonacademic point of view it's probably not.
At an edge of exactly 0% (defining edge as expected return -- equivalent to an edge of 1 as you define it) a Kelly player will be indifferent between betting and not betting (the Kelly stake). For any positive edge (assuming no minimum bet size) the player will strictly prefer to bet. For a bet paying out at 1:1, a Kelly player will choose to bet his edge.
Cool. Out of curiosity what program are you in?
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Welcome ugard. Hope to hear more from you.
(“Program” I took to mean in which university’s economics department are you studying.)
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Maybe a simple example will illustrate:
Assume zero time-value-of-money and full-Kelly. Let's say that today we have a bet at 2.000 with and edge of 20%. This implies a win probability of 60%. Single-period Kelly stake is then 50%. The bet is settles tomorrow night. Tomorrow afternoon, after the other underlying event has already begun, there's a 10% chance there will be a betting opportunity at odds of 2.000 and edge of 90%. This implies a win probability of 100% the opportunity exists. Single-period Kelly stake on that would be 100%.I think you mean to say that you are defining edge as ER + 1, right?
Call the quantity bet on the first event x1 and the quantity bet on the second event x2.
For multi-period optimization we'd have this:
Obviously, that's a pretty contrived example, and could still be solved with standard single-period contemporaneous Kelly (using a 90% push probability for bet # 2), but you should get the idea.Code:maximize U = 90% * [ 60% * ln(1+x1) + 40% * ln(1-x1) ] + 10% * [ 60% * ln(1+x1+x2) + 40% * ln(1-x1+x2) ] wrt x1, x2 subject to a budget constraint of x1+x2 ≤ 1 Solving, we see that utility is maximized at: (x1, x2) ≈ (14.89%, 85.11%).
That's actually untrue. According to Kelly, one would strictly prefer no gamble to a gamble at 0% edge (0% edge => p = 1/o)
It's not an extension, it's just expected utility given log prefs. The stake that maximizes E(U) given log prefs is the Kelly stake.Code:U(no gamble) = ln(1) = 0 maximize U(gamble of x at no edge) = [ln(1+(o-1)*x) + (o-1)*ln(1-x)]/o wrt x subject to 0 ≤ x < 1 U' = [ (o-1) / (1+(o-1)*x) - (o-1)/(1-x) ] / o = 0 implies x* = 0, U = 0 and U'' < 0 for o > 1. Hence U(no gamble) = U(gamble of x at no edge) iff x = 0. and U(no gamble) > U(gamble of x at no edge) iff 0 < x < 1.
I kind of guessed the UK part based on the combination of your proper diction and usage of "behaviour" and "maximisation".I meant in what school's economics department are you studying? Although perhaps you're currently an undergrad?
Last edited by Ganchrow; 03-22-07 at 05:26 AM. Reason: explicitly noted that 0% edge => p = 1/o
SBR Founder Join Date: 8/28/2005
SBR Founder Join Date: 8/28/2005
I'll admit it. I'm lost with this one. I guess I'll just have to assume it has something to do with that vaunted Brit humor I've heard so much about.
Indeed as you've already discovered, the binary choice is but the tip of the proverbial iceberg.
I read back that initial sentence and realized I could have been much more clear. Here's how that sentence now reads (emphasis added), "At an edge of exactly 0% ... a Kelly player will be indifferent between betting and not betting the Kelly stake". Now the Kelly stake at an edge of zero is simply zero so this might appear just some silly restatement of the reflexive property couched in vague economics terms. However, the point of this when considered from the standpoint of continuous preferences is to illustrate that indifference between betting and not betting occurs not at some positive edge, but rather at zero edge. At any arbitrarily small positive edge (ε) the player will always prefer to bet, and at any nonnegative edge, epsilon the player will never prefer not to bet (or "weakly prefer to bet", just to load up on the micro jargon).
Kelly, you'll recall, was not an economist. If an economist had said that I would have thought him at best disingenuous. But from a physicist, especially this physicist, I'm willing to cut the guy some slack.
Popular culture? I just wanted to be like Sid Vicious in high school. He was American, right?
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