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Old 03-25-2009, 03:56 PM   #1
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Probability of flopping a set?

I've read that it is around 12%, but I did a quick check and I came up with a different number. Could someone check my numbers, or explain what I did wrong?

Player starts with a pocket pair in texas hold em. The odds of him flopping a set are Y, odds of not flopping a set are X.

I calculated X using a binomial: Pr(n, k ) = n!/(k!(n-k)!)*pr(k)^k(1-pr(k))^k

I define X as the odds of not hitting either of the 2 cards to improve to a set (or quads, since first you have to have 3 of a kind before improving to 4):
X=Pr(50, 48)+Pr(49, 47)+Pr(48, 46)

First I found Pr(50, 48)=0.276232807
Then Pr(49, 47)=0.27634952
Then Pr(48, 46)=0.276471236

Then X=0.829053564, and Y=0.170946436

This would mean that a pocket pair improves to a set ~17% of the time on the flop, which goes against most of what I googled.
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Old 03-25-2009, 04:19 PM   #2
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Re: Probability of flopping a set?

i dont really have any clue what you've done there. i don't think you understand what the binomial distribution does at all.

the proper way to calculate this would simply be:
(48 c 3) / (50 c 3) = 0.882, which gives you the probability that of the remaining 50 cards in the deck, the flop will contain three of the 48 cards which don't match your pair.

subtract this from 1 to get 0.118, the probability of flopping a set.
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Old 03-25-2009, 04:38 PM   #3
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Re: Probability of flopping a set?

Oh. Yeah I did this wrong, I was finding the odds of something unrelated to the question by thinking that n=number of cards rather than the number of times something is done. Thanks for clearing that up
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Old 03-25-2009, 05:27 PM   #4
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Re: Probability of flopping a set?

you start with a pair (2 cards). there is 50 cards left in the deck, only 2 remaining cards that can give you a set so your odds are 50-to-2 or 25-to-1 chance of flopping a set. 1 divided by 25 is 0.04 which comes out to 4%. so you have a 4% chance of flopping a set.
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Old 03-25-2009, 06:01 PM   #5
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Re: Probability of flopping a set?

Quote:
Originally Posted by TheWuzi View Post
you start with a pair (2 cards). there is 50 cards left in the deck, only 2 remaining cards that can give you a set so your odds are 50-to-2 or 25-to-1 chance of flopping a set. 1 divided by 25 is 0.04 which comes out to 4%. so you have a 4% chance of flopping a set.
You forget that there are 3 cards on the flop
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Old 03-25-2009, 07:23 PM   #6
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Re: Probability of flopping a set?

well the odds of you flopping a set on the turn is still 47-to-2 or little under 24-to-1. 1 divided by 23.5 is 0.04255... which is still 4% lol.
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Old 03-25-2009, 07:28 PM   #7
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Re: Probability of flopping a set?

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Originally Posted by TheWuzi View Post
well the odds of you flopping a set on the turn is still 47-to-2 or little under 24-to-1. 1 divided by 23.5 is 0.04255... which is still 4% lol.
You can't flop a set on the turn. You flop a set on the flop. And you have 3 chances at it - the odds are about 12% as shown above.
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Old 03-25-2009, 11:23 PM   #8
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Re: Probability of flopping a set?

The term 'flopping a set" is a little vague. Does it mean a set and only a set. What about a full house or quads? Anyway, DarkMagnus' answer of 11.8% is for at least a set.
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Old 03-27-2009, 04:59 AM   #9
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Re: Probability of flopping a set?

Quote:
Originally Posted by Illini10 View Post
I've read that it is around 12%, but I did a quick check and I came up with a different number. Could someone check my numbers, or explain what I did wrong?
You will get a PP 5.88%
Now, that you've got that pair you will flop a set or better 11.7%

In more more practical terms, what to expect in real live game:
You will flop 11.7 sets for each 100 PP that you hold. In order to get 100 PP you will have to "play" 1,700 hands.

As you can see, you will have a set on the flop once per (1,700*11.7)/100=199 hands. Now, how many hours takes you to play 199 hands in live poker? Well, you have 199/33=6 hours. On average you can say that you will have at least a set on the flop once every 6 hours of live play. If you game is much faster than 33 hands/hour then you tune 199/(# of hands per hour)

You can associate the set frequency with the frequency of getting a specific premium PP like AA. As we all know, we will get AA once in 220 hands.

So, I would say this: In 6-7 hours of live play we can expect on average to get pocket AA once and besides that to flop some kind of any set also. In a full day of play you can see that you’ll get on average the supreme PP plus some kind of set. We will have at least two big hands during one day of live play. But don't go crazy with your AA after the flop. You can shove all-in preflop if you have the opportunity but be cautious post flop with unimproved AA.

Last edited by Octavian; 03-27-2009 at 05:15 AM.
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Old 03-27-2009, 05:43 AM   #10
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Re: Probability of flopping a set?

To flop a set (or better):
You know what 2 cards are because you have them as a pair in your hand. First card off the deck then you have a 2/50 chance to hit. (A)
If that fails you will have a 2/49 chance to hit on the second card (B)
If that fails you will have a 2/48 chance to hit the last flop card. (C)

As you aren't bothered (from the assumption at the start) whether you hit a set, quads or a boat, the maths then becomes [A OR B OR C] which is [2/50 + 2/49 + 2/48] which comes out at a little worse than 7/1 or the 12% previously quoted.

There are lots of situations (particularly live play where you don't have your calculator handy) where a rough and ready calculation will do - so [2/50+2/50+2/50] = 6/50 = 12%. You get a small rounding error, but moderate maths ability could get you there during a hand if you wanted to know if you were getting your price.

If you are playing a hand and there is a pot of $585 and it is $150 to call, for all but the very gifted the decimal places are immaterial as you can't work it out in your head and it shouldn't make a difference anyway if the pot is laying you 12% OR 12.2% Even if you are capable of working it out, it is so close that the difference shouldn't be the difference for you between one play and another. Knowing that you are 'a bit worse than 4/1' from the pot should be all the maths you need to play the hand and all the other factors can come into play.
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Old 03-27-2009, 09:38 PM   #11
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Re: Probability of flopping a set?

Relevance being, most weak players can see flushes or straights on a flop but a set catches them off guard. How big of a raise will you call with a pocket pair under 9s in hopes of flopping a set? The implied odds in low limits are huge.

Last edited by Zeds_Dead70; 03-27-2009 at 09:39 PM. Reason: grammage
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Old 03-27-2009, 11:15 PM   #12
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Re: Probability of flopping a set?

Quote:
Originally Posted by Zeds_Dead70 View Post
Relevance being, most weak players can see flushes or straights on a flop but a set catches them off guard. How big of a raise will you call with a pocket pair under 9s in hopes of flopping a set? The implied odds in low limits are huge.
With a big pair like AA, KK, QQ I will raise if Iím the first one in or 3bet if opponent raises upfront. Now if I play deep stack, and have PP under 99, I will call based on stack odds. If the raiser is deep too I will call 5% to 7% of my stack. If for example I have $1,000 and pocket 77, the guy up front open raise for $50 to $75, I will call if I figure Iím not going to be re-raised in the back. I call up to 7% of my stack and see the flop even if I suspect the upfront raiser has AA.

In fact, I want him to have AA and I want him to believe that he should play that pair real fast and hard all the way to the river. That is the ideal scenario for your 77. If the raiser bets about 1/15 of his stack and you cover than I would say CALL. If on the other hand the raiser has $100 stack and he open-raise for $25 and you suspect him for having a big pair or AK I will not call. Why? Because you will be a big dog against any overpair and about even money against AK and the reward when you hit your set will not cover the losses you accumulate when you miss. He doesnít have enough dough for you to attack after the flop.

When you hit your set on the flop and there is a flush or a straight draw possible, donít rush to raise and re-raise to knock your opponent out of his draw. Just bet enough for him to call as a mistake but do not raise him too much so heís making the correct play of folding. Bet about halve of the pot. You want him to call you! You have more outs to make a FH or Quads against him. When you have a set on the flop and his drawing to flush heís got only 7 clean outs while youíve got the rest of the deck working for you. Actually, from 47 cards left heís got 7 clean outs to flush while you donít have to hit anything to beat him unless he hits his flush, after that point you still have 10 outs to kill his hand.

Never slow play a flopped set! Nobody knows you have a monster anyway, therefore, it is stupid to conceal the strength of your hand. Backing off to a raise and then check-raising on the turn is a valid strategy (although not necessarily best). I do not back off when there is a third suited card on board. I feel that I have enough outs to disregard the possibility of a made flush against me.

Note:
1. If you lose with a set, you'll lose a lot of money. If you don't, you are not playing your sets correctly

2. The reverse itís also true: If you win with a set, you'll win a lot of money. If you don't, you are not playing your sets correctly

Last edited by Octavian; 03-27-2009 at 11:30 PM.
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Old 03-28-2009, 10:27 AM   #13
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Re: Probability of flopping a set?

Quote:
Originally Posted by SimonG View Post
To flop a set (or better):
You know what 2 cards are because you have them as a pair in your hand. First card off the deck then you have a 2/50 chance to hit. (A)
If that fails you will have a 2/49 chance to hit on the second card (B)
If that fails you will have a 2/48 chance to hit the last flop card. (C)

As you aren't bothered (from the assumption at the start) whether you hit a set, quads or a boat, the maths then becomes [A OR B OR C] which is [2/50 + 2/49 + 2/48] which comes out at a little worse than 7/1 or the 12% previously quoted.

There are lots of situations (particularly live play where you don't have your calculator handy) where a rough and ready calculation will do - so [2/50+2/50+2/50] = 6/50 = 12%. You get a small rounding error, but moderate maths ability could get you there during a hand if you wanted to know if you were getting your price.
Your approach has some errors.

To compute the probability that you will hit a set or better, then using a Venn diagram, you can set the problem up like this:

Deal the cards one at a time and label the first card A, the second card B, and the third card C.

P(A or B or C)

= P(A) + P(B) + P(C) - P(A and B) - P(A and C) - P(B and C)

P(A or B or C)

= 2/50 + 2/50 + 2/50 - 2/50*1/49 -2/50*1/49 - 2/50*1/49

~ 11.755%
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Old 03-28-2009, 03:01 PM   #14
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Re: Probability of flopping a set?

The easy way of doing this is by starting with the approximation that the chances of a particular card coming up is about 2%.

So if there are two cards that can help you and there are three cards flipped up, the chance is 2(good cards)x2(%)x3(flop)=12%.

Let's delve into this even deeper. Say you have AKoff. You now have 6(good cards)x2(%)x3(flop)=36%.

Thank you, thank you, I'll be here all week.
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Old 03-29-2009, 10:28 AM   #15
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Re: Probability of flopping a set?

Quote:
Originally Posted by jay_shark View Post
Your approach has some errors.

To compute the probability that you will hit a set or better, then using a Venn diagram, you can set the problem up like this:

Deal the cards one at a time and label the first card A, the second card B, and the third card C.

P(A or B or C)

= P(A) + P(B) + P(C) - P(A and B) - P(A and C) - P(B and C)

P(A or B or C)

= 2/50 + 2/50 + 2/50 - 2/50*1/49 -2/50*1/49 - 2/50*1/49

~ 11.755%

To me all it looks like you have done here is worked out a set or better, and then subtracted the probabilities of hitting a better hand. So your answer looks correct for Probability of hitting a set and only a set. In poker, whilst you are working out your price of flopping a set, you presumably won't mind hitting quads either, so the assumption of "set or better" seems fair.

No matter either way, most of my point is that whether you use a quick and relatively accurate method such as the 2% rule - it is something you can do real time and is therefore helpful to live poker players. Real time, Venn diagrams aren't any use, the maths is interesting from a 'settling a prop bet' angle to 4 decimal places, but for poker, quick and approximate ftw.
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