50/50/90 rule - do I hear people snicker?

123enen · 3 May 2006

Did you know that the 50/50/90 rule is real?
This is how it works, but first I need to explain the 50/50/75 rule.

50/50/75 rule

Four playing cards are laid face down on a table.
One of them is the Ace of diamonds.
Your job is to nominate the card that you believe is the ace of diamonds.

After you have made your choice the moderator turns one of the three other cards face up.
It is not the Ace of diamonds.
The game continues.

Another card is turned face up
It is not the Ace of diamonds.
The game continues with two card left face down on the table.
One of them is the Ace of diamonds, the other is not.

You are now given an opportunity to change your selection.
A 50/50 choice.
Do you stay with the original selection or do you change cards.
Think about it, make your choice then continue on.

Remember, the original card you chose had a 75% chance (1 in 4) of being the wrong card. There was a 75% chance that one of the other three cards was the ace of diamonds.
The game has done you a favour. It has exposed two of those other cards, and now tells you that out of TWO remaining cards, ( 50/50 chance) one has a 75% chance of being the Ace of diamonds and the other, your original selection, only has a 25% chance.

That is the 50/50/75 rule.

The 50/50/90 rule; start with 10 playing cards. The one you choose has a 90% chance of being wrong. What do you do when you get down to two cards.

ctp6360 · 3 May 2006

wow that just blew my mind, that's a great post thanks!

GreatPig · 3 May 2006

123enen said:
out of TWO remaining cards, ( 50/50 chance) one has a 75% chance of being the Ace of diamonds and the other, your original selection, only has a 25% chance.

That's crap. Both cards have a 50% chance of being the ace, so it makes no difference whether you change your selection or not (in terms of the long term average).

Your selection only has a 25% chance of being the ace when all four cards are present. The moment one is removed, the probability changes.

GP

WaySolid · 3 May 2006

Looks like a variant of the 3 door gameshow puzzle.

It should be stated that the person turning over the cards knows the location of the Ace of Diamonds and deliberately does not turn it over.

Yes you should switch as it definitely improves your odds although intuitively it's very difficult to believe this.

slackjaw · 11 August 2008

WaySolid said:
Yes you should switch as it definitely improves your odds although intuitively it's very difficult to believe this.

The dealer deliberately didnt turn over the Ace because he knew where it was, or because he couldn't, because it was the card you chose. Both are of equal probability. This aint even high school maths. You're down to 2 cards, regardless of which of these you chose to begin with each has the same probability of being the Ace.

If intuition played any part, then you would have chosen the correct card in the first place, so intuitively, your odds might be better if you dont change

WaySolid · 11 August 2008

slackjaw said:
The dealer deliberately didnt turn over the Ace because he knew where it was, or because he couldn't, because it was the card you chose. Both are of equal probability. This aint even high school maths. You're down to 2 cards, regardless of which of these you chose to begin with each has the same probability of being the Ace.

If intuition played any part, then you would have chosen the correct card in the first place, so intuitively, your odds might be better if you dont change

Intuition doesn't imply magical forecasting ability.

ghotib · 11 August 2008

Umm. Why didn't each of the first 4 cards have a 1 in 52 chance of being the ace of diamonds? The setup didn't say it was definitely there at all.

Think I'll stick to Lotto.

2020hindsight · 11 August 2008

Lol
It's like the old blokes at the roulette wheel -
you get 5 reds in a row, they'll swear that the chance of a black next is improved by this "sequence-of-past-events" , i.e. is better than 50-50 (ignoring the green for a moment)

(PS or ignoring the two greens !!, if you're stupid enough to play roulette in Vegas

pepperoni · 11 August 2008

*snickers*

Buster · 11 August 2008

Hmmm..

I always thought that it was to explain why it is that whenever I am given a 50/50 opportunity, I'll pick incorrectly and get it wrong 90% of the time..

Regards,

Buster.

slackjaw · 11 August 2008

WaySolid said:
Intuition doesn't imply magical forecasting ability.

http://dictionary.reference.com/search?q=intuition

2020hindsight · 11 August 2008

Buster said:
I always thought that it was to explain why it is that whenever I am given a 50/50 opportunity, I'll pick incorrectly and get it wrong 90% of the time.. .

buster,

something like murphy's law ?
drop the toast, it always lands butter side down

PS Murphy was an optimist

Buster · 11 August 2008

Hey 2020..

2020hindsight said:
something like murphy's law ?
drop the toast, it always lands butter side down

Ha ha.. Well ALMOST always..

Regards,

Buster

Lubenc · 11 August 2008

Sorry slackjaw but you are wrong.

Say there were 10 cards and one is an ace.

When you get down to the last two cards there are two possiblities either:

1. the card you chose (when there were 10 to choose from) is the ace
2. the other card is ace

The odds that the card you chose initially was the ace are 1 in 10

The odds you chose a card other than the ace are 9 in 10

So, some may say counter-intuitively, the odds are significantly greater
that you will end up with the ace if you switch.

2020hindsight · 11 August 2008

anyone know the formula(e) they use on the "deal or no deal" show?
I've only seen it a few times (and certainly haven't cracked the code).

Maybe something like - "the mean of the remaining suitcases less 10% ??"

white_goodman · 11 August 2008

2020hindsight said:
anyone know the formula(e) they use on the "deal or no deal" show?
I've only seen it a few times (and certainly haven't cracked the code).

Maybe something like - "the mean of the remaining suitcases less 10% ??"

im sorry i cant watch it andrew okeefe is a douche

korrupt_1 · 11 August 2008

2020hindsight said:
anyone know the formula(e) they use on the "deal or no deal" show?
I've only seen it a few times (and certainly haven't cracked the code).

Maybe something like - "the mean of the remaining suitcases less 10% ??"

:topic

so... after all but two cases remain... the 50c and $200k...

your chance of getting the $200K is 50/50...

what will the bank offer you? $99,995? i dont think so..... im guessing it will be $80,000 or something like that....

i think the bank takes human greed and emotion into it...

what would u do... take the unfair offer of $80K or take a chance at 50/50... knowing you could go home with 50c...

remember, the bank KNOWS what you have... much like the card dealer knew where the Ace of Diamond was...

2020hindsight · 12 August 2008

white_goodman said:
im sorry i cant watch it andrew okeefe is a douche

that sounds like a variation of ad hominem mate

korrupt_1 said:
so... after all but two cases remain... the 50c and $200k...
your chance of getting the $200K is 50/50...

im guessing it will be $80,000 or something like that....

what would u do... take the unfair offer of $80K or take a chance at 50/50... knowing you could go home with 50c...

ahhh - now we're getting down to the short strokes here korrupt .

I'd reason as follows ...
You have the option to go on ... with a negative casino advantage !!
There is $200,000.50 out there - On average (if you did it 100 times whatever) you will go home with $100,000.25.

And they are only offering $80,000.

When next in your life will the odds of a positive outcome be in YOUR favour rather than the bank's ?

PS lol, then again - perhaps you're right - one bird in a penthouse or two birds in the bush, whatever that saying is

2020hindsight · 12 August 2008

It's like the odds bet at the craps table ..

When you play craps, the casino advantage for bets on the pass line is 1.4% i.e. for every $100 bet, the casino keeps $1.40 in the long run. If a number comes up first roll ( other than a natural or a crap out), you can then place an odds bet behind your first bet of at least the same value - and the casino gives you perfect odds. i.e. suppose the first roll is a four, and the chances of you winning (getting a 4 before you get a seven) is 1:2, then the casino pays you 2:1. It is a GIFT!! - one of the few in the casino , other than the cup of lousy coffee that the girl in the skimpy dress brings you

(i.e. 3 ways for two dice to come up with 4 , = 1+3, 2+2, 3+1
6 ways for them to come up with 7, = 1+6, 2+5, 3+3, 4+3, 5+2, 6+1
casino pays you 2:1 = perfect odds, ok)

So my question then is ... if you can't afford to put the odds bet down, - because you claim that's like putting all your eggs into one basket sort of thing - should you pack up and go home? - or hang around for the next chance to bet with 1.4% casino advantage?.

I'd answer that you'd be mad to hang around. If you can't afford to bet when the casino is effectively GIVING money away, (- correction - make that GIVING you free entertainment lol) then you can't afford to bet period.

PS Compare that to roulette wheel with 1/37 = 2.7% casino advantage (in Aus)
or 2/38 = 5.3% in Vegas where there is a "00" as well as an "0" - sheesh. now that is crazy.

Albert Einstein is reputed to have stated, "You cannot beat a roulette table unless you steal money from it."

https://www.aussiestockforums.com/forums/showthread.php?p=175398

Say Vegas permits odds bet of 3 times your first bet, (although Reno is better, up to 10)

and you have choice of either .......

a) craps table, betting one $10 chip per roll, and $30 odds bet behind it - and hence you are betting, each hour, about
102 rolls x $10 and in addition about 34 odds bets at $30 = say $2000 per hour, (casino advantage approx 0.47% based on post #11)

b) or roulette wheel, betting at the same rate, eg very approx $70 per roll x 30 rolls of the "little wheel" per hour = $2000 per hour (with "0" and "00", as they do in US, casino advantage is (2 / 38) = 5.26%!! ridiculous)

THEN
a) the craps would cost you $14.40 per hour,

b) and the roulette would cost $105.20 per hour !! (7.3 times more expensive.)

SUMMARY - you can bet for a week playing craps in Vegas, for the same probable loss as betting at the same rate at the roulette wheel - but for 1 day!!.

2020hindsight · 12 August 2008

2020 said:
Say Vegas permits odds bet of 3 times your first bet, (although Reno is better, up to 10)THEN
a) craps would cost you $14.40 per hour,

oops, but Aus is more expensive of course ( not permitting such large odds bets.)

50/50/90 rule - do I hear people snicker?

123enen

ctp6360

GreatPig

Pigs In Space

WaySolid

Learner

slackjaw

WaySolid

Learner

ghotib

THIMKER

2020hindsight

pepperoni

Buster

slackjaw

2020hindsight

Buster

Lubenc

2020hindsight

white_goodman

BOC

korrupt_1

2020hindsight

2020hindsight

2020hindsight

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