Risk of Ruin formula question

London Colin

Well-Known Member
#1
Is the formula given on page 112 of BJA3 completely general purpose ? (i.e. equally applicable to any game, not just blackjack.)

The formula is RUIN = ( (1 - w/sd) / (1+w/sd) )^(bank/sd)

where w and sd are said to be the 'hourly' win rate and standard deviation, but presumably can be for any period, including per-hand, so long as they both match?

With my limited mathematical background, I don't know how the formula was derived (and might well struggle to understand the explanation in any case :)).

I've been working on a simulator which logs the win rate and std dev for various side bets (both individually and in combination). In any given round there may be -
  • no bets made
  • one or more different +EV bets made, with a compulsory -EV main bet
  • one or more different +EV bets made, with no compulsory main bet
And the different bet types available have different payoffs and may have different min and max bets, making the definition of a 'unit' seem a little nebulous to me. (I suppose it would have to be the smallest of all the available bets, if it has any meaning at all.)

I'm hoping the above ruin formula can be used to analyse different betting schemes, based purely on the accumulated results and squared results from the simulation. Am I on the right track?
 

sagefr0g

Well-Known Member
#2
you think you have a limited mathematical background? you oughta try being me :(, diffy Q's is far as i ever got, trigonometry & analytic geometry was a nightmare for me, never had a probability course but was made to figure some sort of quantum position of an electron relative to a proton in a hydrogen atom, that with out even having a clue as to what probability was, got an A in that course and hadn't a clue what i was doing.
but anyway, i've used that formula, for loads of stuff (all kinds of games results, bunched together) other than blackjack. do i know if it's a valid use of the formula? nope. but what i do know is what seems to be reasonable results of that use.
point being, why not give what you are hoping for a try?
after trying, see if it seems to make sense.

below (note: the page reference next to ROR bottom of sheet and the formula shown in the top of the picture) is something for which the formula seems to work:

 

gronbog

Well-Known Member
#3
Yes, the formula is general, not just for blackjack and yes, the specification of hourly rates is arbitrary.

You enumerate 3 possible scenarios.
London Colin said:
  • no bets made
  • one or more different +EV bets made, with a compulsory -EV main bet
  • one or more different +EV bets made, with no compulsory main bet
  • The first is like wonging out in blackjack. Treat it like a bet of zero with EV=0 and SD=0. This will allow you to get overall results for "rounds observed", which should be used calculate results over various time periods (hourly, weekly, monthly, etc.). Ignoring this scenario gives you results for "rounds played" which can also be useful.

  • For the second, use the initial compulsory bet as the unit. You want your results in terms of that initial wager. We do the same in blackjack where your initial bet advantage (or IBA or EV) are expressed in terms of your initial wager, even thought you may wager more in order to double, split and/or take insurance.

  • For the third scenario your unit can bet anything you like. Treat each +EV bet like a separate game.
Once you have the results for each scenario, you can combine them based on the frequencies of each scenario. Be careful here, you can multiply EV by frequency, but not so with SD. You need to multiply that by the square root of the frequency.

Have fun!
 

gronbog

Well-Known Member
#4
I just thought about this some more. If the bets are all independent, then I think you can get everything you need like this:
  1. analyse all of the bets separately to get the EV and SD of each (the main bet and all of the side bets)
  2. determine the frequency of each combination of bets you might make
  3. determine the EV and sd of each combination of bets you might make
    • you can add EVs
    • you can't add SDs, but if the bets are independent, you can add the variances (each SD squared) and then take the square root of the result
    • don't forget to weight each EV and SD by the relative size of each bet
  4. Get the overall EV by multiplying the EV of each combination by it's frequency and adding the results
  5. Get the overall SD by multiplying the SD of each combination by the square root of its frequency and adding the results
 

London Colin

Well-Known Member
#5
gronbog said:
Yes, the formula is general, not just for blackjack and yes, the specification of hourly rates is arbitrary.
Thanks for confirming that. I thought it was probably the case. The one thing that was giving me pause was that I'm looking at games where the advantage can get a lot bigger than in blackjack, which might have caused the formula to break down if it relied on some unspoken assumptions.

gronbog said:
You enumerate 3 possible scenarios.

  • The first is like wonging out in blackjack. Treat it like a bet of zero with EV=0 and SD=0. This will allow you to get overall results for "rounds observed", which should be used calculate results over various time periods (hourly, weekly, monthly, etc.). Ignoring this scenario gives you results for "rounds played" which can also be useful.
Yeah, I was already thinking along those lines, though getting a bit confused in the process, considering whether rounds observed, rounds played, or both would be the desired/most meaningful input into calculations such as risk of ruin. As with the choice of hourly rates or other periods, I guess the important thing is merely to be consistent.

The raw data that I am currently collecting is essentially -
  • Num rounds observed.
  • Num rounds played (i.e. one or more bets made on the round)
  • For each separate bet type and in total -
    • Num bets made.
    • Total amount bet.
    • Sum of results.
    • Sum of squared results. (where each result is the net profit on a round)
- which should be enough to let me extract lots of useful information.

Since a simulation will involve varying bet size with advantage, presumably I can only get figures for basic, 1-unit std deviations if I either set up special flat-betting runs for that specific purpose, or alternatively add some additional counters to keep track of how many of each possible payoff there have been.

It's not really relevant to this discussion, but I am also logging the above data by card penetration, as well as in total, so that I should be able to run a simulation once, configured for the maximum possible pen., then query the results for shallower penetrations by simply excluding the unwanted data, rather than having to run the sim again.


gronbog said:
  • For the second, use the initial compulsory bet as the unit. You want your results in terms of that initial wager. We do the same in blackjack where your initial bet advantage (or IBA or EV) are expressed in terms of your initial wager, even thought you may wager more in order to double, split and/or take insurance.
It's not quite the same thing, as the bet is only compulsory in the sense that it has to be made alongside the side bet(s) that we really want to make (because they have gone +EV). It's the normal situation with side bets; it just so happens that in some games one is allowed to make 'side' bets without being required to make the main bet at all.

I'm still not sure if thinking in terms of units is really necessary at all, compared to just dollars or pounds.

gronbog said:
  • For the third scenario your unit can bet anything you like. Treat each +EV bet like a separate game.
Once you have the results for each scenario, you can combine them based on the frequencies of each scenario. Be careful here, you can multiply EV by frequency, but not so with SD. You need to multiply that by the square root of the frequency.

Have fun!
I've noticed you've commented further while I've been typing!:) I'll save any further thoughts I was going to add here until I've digested everything.

Thanks a lot.
 
Last edited:

London Colin

Well-Known Member
#6
gronbog said:
I just thought about this some more. If the bets are all independent, then I think you can get everything you need like this:
  1. analyse all of the bets separately to get the EV and SD of each (the main bet and all of the side bets)
  2. determine the frequency of each combination of bets you might make
  3. determine the EV and sd of each combination of bets you might make
    • you can add EVs
    • you can't add SDs, but if the bets are independent, you can add the variances (each SD squared) and then take the square root of the result
    • don't forget to weight each EV and SD by the relative size of each bet
  4. Get the overall EV by multiplying the EV of each combination by it's frequency and adding the results
  5. Get the overall SD by multiplying the SD of each combination by the square root of its frequency and adding the results
Some of the bets I am currently interested in are definitely not independent (e.g., they have mutually exclusive outcomes), and others could be independent but could also be correlated in ways that I haven't even considered.

To be honest, I hadn't really considered trying to combine sim results (EVs, SDs, etc.) for selected individual bet types, in order to answer questions like "What would happen if I played a game offering bets of type A, B, and C?"

Instead, my planned way of answering such a question was to run a sim with just those three bets available, and track the net result of each round. But in doing so, there is obviously scope to log some useful information about the individual bet types too.

Does that make sense?
 

gronbog

Well-Known Member
#7
Logging results by current penetration (by card) is a good idea if you have enough resources.

You can sim the bets the way you intend to make them and get an overall result. The nice thing about combining separate results is that you can do things like computing the optimal bet sizes for each under various conditions which would then allow you to construct optimal ramp combinations and to compute the global win rate and RoR for various ramp combinations in a CVCX-like way.
 

London Colin

Well-Known Member
#8
gronbog said:
The nice thing about combining separate results is that you can do things like computing the optimal bet sizes for each under various conditions which would then allow you to construct optimal ramp combinations and to compute the global win rate and RoR for various ramp combinations in a CVCX-like way.
I can see that would be very useful; I'm just unsure if it is possible, given the correlation issue.

If it is possible, hopefully it is a feature I can bolt on at a later date. I think I need to keep things simple to begin with!:)
 
Top