Something about probablility

**Jeremy G** · 03-10-2008

Originally Posted by mike_g

I was talking to this guy that loves gambling the other day, and he had this theory about how to win lots of money.

Using a roulette example (ignoring 0 for simplicity sake) he said that each time one colour follows a colour of the same type the probability of the ball landing on the opposing colour doubles. EG: 3 blacks in a row means that on the next spin theres a 1:8 probability that by betting red you would win.

I cant see how that would work. The way i see it is that you would still 2:1 odds (or perhaps only a tiny fraction less) as the result would be relevant to the point in time that you place the bet.

His argument was that it was due to the law of averages, which to him meant that he was right. AFAIK the law of averages was made up to measure the behavior of atoms when there was no accurate way of doing so and only really applies to orders of great magnitude.

Anyone know whats right here?

The method he is hitting on is called progressive gambling. It is true that for any single instance of the roll, neglecting greens, its near 50/50 change to land on one particular color. However, the odds for successive same color results decrement with each spin. The way to win, is to bet twice the money you bet on the previous round.
Round One:
bet $1 on red.
Lands on black-.........
Round Two:
bet $3 on red.
Lands on black-.........
Round Three:
bet $5
Lands on red-yey. Collect $10 dollars, you made $1.
Granted, there is a possibility to continue rolling blacks, however it is unlikely--and thats what you are betting on. The only way to lose is to lose more than half your money.

**Oysterman** · 03-10-2008

Originally Posted by Jeremy G

The way to win, is to bet twice the money you bet on the previous round.

Although that is not what you had in your example.

That approach is however referred to as the Martingale system, a google-friendly term.

The only way to lose is to lose more than half your money.

Or you run into the house's betting cap, with happens quite soon if your bet increases exponentially. Martingale is actually a sure way to lose money in the long run in real life scenarios (not in absurd theoretical ones with infinite bankrolls and no betting cap).

**Sang-drax** · 03-10-2008

Originally Posted by novacain

Tell his guy he should bet WITH the run, not against it.

This is because the run (of the same colour) can continue (theoretically) forever. The run can end only once (and he has to pick that one spin).

Oh, come on! The odds of winning are exactly the same, no matter if you bet "with" the run or "against" it.

**novacain** · 03-10-2008

Table maximums are carefully calculated to stop the Martingale system.

Originally Posted by Sang-drax

Oh, come on! The odds of winning are exactly the same, no matter if you bet "with" the run or "against" it.

Sure, each event is independent according to probabilty.

But runs of luck happen, quite frequently IME.

Many follow the run to see how far it goes, then move tables to find the next run.

Best to follow the run until it ends (winning each time it continues), rather than betting it will end (losing each time it continues).

I did not totally believe in (random) luck until I spent nearly three years dealing at a casino. Some days I could not fail to draw the cards I needed to win (and knew it), others I was giving out cash as fast as they could bring it to the table.

**brewbuck** · 03-10-2008

Originally Posted by novacain

Table maximums are carefully calculated to stop the Martingale system.

Yeah, it's funny how people think they're smart for coming up with such systems. After all, a casino with billions of dollars to lose is obviously too dumb to see the truth. This isn't even a question of probability, but simple economics.

Many follow the run to see how far it goes, then move tables to find the next run.

That makes about as much sense as predicting my lifespan by counting how many eyelashes I have.

I did not totally believe in (random) luck until I spent nearly three years dealing at a casino. Some days I could not fail to draw the cards I needed to win (and knew it), others I was giving out cash as fast as they could bring it to the table.

It is human nature to observe when events deviate from their probabilistic averages. You remember a few times when you gave out tons of cash. What you don't remember so vividly are the days (and I guarantee there are much more of them) when that DIDN'T happen.

Same thing with "I was just thinking about you, and at that very instant you call me on the phone." Sure, seems weird, but you discount all the times you were thinking of that person and they DIDN'T call you. Or the times they called you yet you were not thinking about them. These events are "normal" and we don't remember them.

**Mario F.** · 03-10-2008

Originally Posted by brewbuck

It is human nature to observe when events deviate from their probabilistic averages. You remember a few times when you gave out tons of cash. What you don't remember so vividly are the days (and I guarantee there are much more of them) when that DIDN'T happen.

Selective memory is what gives casinos their money.

**Sang-drax** · 03-11-2008

Originally Posted by novacain

Best to follow the run until it ends (winning each time it continues), rather than betting it will end (losing each time it continues).

No. no. no. It. does. not. matter.

**novacain** · 03-11-2008

Originally Posted by Sang-drax

No. no. no. It. does. not. matter.

Actually it does.

You are assuming the table / game produces PERFECT results every time, all the time. That is that each event is exactly equal in probability always.

In real world applications this is simply not true. A slight mis-alignment in the table can cause a change in the probabilities on the wheel.

Casinos rebalance all AR wheels on a weekly basis (IME) because they do not stay 'true' for long in the casino environment.

**Daved** · 03-11-2008

That may be true, but the chance that a particular run of luck is due to a slight variation is so small that it isn't even worth paying attention to.

If somebody told me that a coin had a 50.00001% chance of being heads and 49.99999% chance of getting tails, sure I'd choose heads. But if you only knew that it was 50.00001/49.99999 and you didn't know which way the bias was, you can't assume that 5 tails in a row means the bias is towards the tails.

So even if it does matter, it matters so little that statistically speaking it doesn't matter.

**novacain** · 03-11-2008

Joseph Jagger/s would not agree with you.

Casinos do not spend time and money when it is not needed.

**Daved** · 03-11-2008

The odds of you finding such a scenario and being able to understand it well enough to take advantage of it and still not get caught until you've made money on it are astronomical. It's not worth the effort.

And the other point is that when you see a run following a certain pattern, the chances are still astronomical that it is due to a real bias rather than normal variation.

**Sang-drax** · 03-13-2008

Originally Posted by novacain

Actually it does.

You previously said: "Tell his guy he should bet WITH the run, not against it." and "Many follow the run to see how far it goes, then move tables to find the next run." There is just nothing to gain here, as Daved explained.

The topic was Roulette strategies and betting "with the run" or "against the run" does not make a difference. To find differences in a Roulette table, you'd have to observe millions of runs and then play a million times to exploit it.

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