[ 3 / biz / cgl / ck / diy / fa / ic / jp / lit / sci / vr / vt ] [ index / top / reports ] [ become a patron ] [ status ]
2023-11: Warosu is now out of extended maintenance.

/sci/ - Science & Math

View post   
File: 100 KB, 1000x1939, 1277509359162.png [View same] [iqdb] [saucenao] [google]
2839253 No.2839253 [Reply] [Original]

You have 3 doors. A car and 2 goats are behind them. You pick door #1, and Monty Hall opens door #2, revealing a goat. What are the odds that door #1 has a goat?

Oh and by the way Monty has this weird smirk on his face, and his motivation for opening door #2 is unknown.

Not so easy anymore, is it?

>> No.2839259

33%
/thread

>> No.2839269

You're right, human expression throws probabilistic analysis out the window.

He's probably smirking because he knows you're a /sci/duck and you'll pick door #3 because you've heard this problem a million times.

>> No.2839273

>>2839259
If door #2 had opened accidentally, you wouldn't still say 33%. (Or would you?) So if you have no idea why he decided to open that particular door, how is that different from it happening entirely randomly?

>> No.2839278

Just because he has a weird smirk doesn't mean that the stats change.

>> No.2839291

>>2839278
http://en.wikipedia.org/wiki/Monty_Hall_problem

>> No.2839306

>>2839291
I don't see what you're trying to say.

>> No.2839315

Group A is Door 1, which has a 33% chance of a car.
Group B is Door 2 and 3, which has a 66% chance of a car.
There is definitely a goat in Group B.
There is a 100% a goat will be revealed.
So, when you switch, you get both Door 2 and 3 as choices, so the odds of the car are better.

That's my understanding at least.

>> No.2839330

>>2839315
>There is a 100% a goat will be revealed.
I don't know. Remember that smirk? Kind of unnerving.

>> No.2839333

you always switch.

>> No.2839337
File: 205 KB, 689x800, 1284403529645.jpg [View same] [iqdb] [saucenao] [google]
2839337

>>2839291
>>2839315
>>2839259
>missing OP's point

>>2839273
Why would it open accidentally? I would assume the people who build the set are competent enough to make a door that closes.

>> No.2839339

door #2 had the goat so 0

>> No.2839363

This isn't the original problem. I think it's 'you decide to open door 1, but then Dorkman opens door#2 and then asks you if you want to change doors. You have to decide if there's a real mathematical advantage to switching.

This is a real problem, with a real math-related solution. No 'smirks.'

>> No.2839384

troll thread

Montys 'weird smirk'/motivation is irrelavent because you picked first. 2 doors, 2 objects. 50/50.

>> No.2839385

>>2839363
'you decide to open door 1, but then Dorkman opens door#2 and then asks you if you want to change doors. Then he flashes you his dick. do you change?'

>> No.2839396

>>2839363
>This is a real problem, with a real math-related solution. No 'smirks.'
So incomplete understanding of human motivations makes mathematical analysis worthless?

>> No.2839426

They had this problem in a movie, it made no sense as to why the probability would change then either.

>> No.2839455
File: 25 KB, 640x400, aaaaa.jpg [View same] [iqdb] [saucenao] [google]
2839455

draw a fuckin picture this shit is easy.

>> No.2839485

33%

You don't need to know his motivation for opening door #2, or that he has a weird smirk on his face. That's neither qualitative or quantitative information.

Originally you had 66% chance of picking a goat, and 33% chance of picking a car. When monty reveals the goat, that changes the odds that you picked the goat to 66%.

>> No.2839572

>>2839485
see
>>2839273
50/50.

>> No.2839595

I'm really dumb so explain to me why it wouldn't be 50/50? two doors left - one has a goat, one has a car so...

>> No.2839884

>>2839595
It is 50/50.

In the normal version of the problem, Monty's forced to reveal a goat, which makes it 2/3, 1/3. But if you don't know what his intentions were in opening the door, you just have to treat it like a random event -- not affected by your choice of door #1, in other words. So it's 50/50.

Unless there's a way of averaging over all possible mental states...

>> No.2841284

bump

>> No.2841301

>>2839273
Accidental or not doesn't make a difference. There's still one less goat than there used to be, meaning that there's a diminished chance that the remaining one not opened, accidental or not, is a goat.

>> No.2841299

2/3

>> No.2841307

>>2841301
There is also one less door than there used to be, which means the chance is levelled out.

>> No.2841308
File: 505 KB, 640x480, 1299554563811.png [View same] [iqdb] [saucenao] [google]
2841308

>>2839455

>> No.2841317

>>2839253
100% there's a goat, because of the smirk

>> No.2841319

How the fuck was this not resolved in under one post?
>oh wait, it was.

So why are people still posting?

>> No.2841320

>>2839253

In cold logic, disregarding socio-emotional trickery, the ods are 50/50.

The question would be more of why he chose door 2 instead of door 1 even though you asked for door 1. The smirk of Monty can be Interperted in two ways: Either the goat or the car could be behind door 1.

Either way, ods still are 50/50 despite the socio-emotional trickery.

>> No.2841330

When you picked a door, you had a 1 in three chance of picking the car. There was a 2 in 3 chance the car was behind the other two doors.

When he reveals a goat, that 2 in 3 chance then gets compressed to the remaining door. Verified by math many, many times.

Most people who try to explain why this is (including Wikipedia) give an invalid explanation that just makes it more confusing.

>> No.2841414

Can anyone who claims it's 2/3 explain why this is wrong:
>>2839273

>> No.2841428

the point here--and it's a good one--is the "always switch" solution to the monty hall problem ONLY works if monty HAS TO open a door no matter what. if he is free to open a door or NOT open a door, then he is free to open a door only when you have picked the door with the car. and if he opens a door only when you have picked the door with the car, then the obvious solution is NEVER switch.

>> No.2841431

The chance for a goat at the initial pick is 2/3.
The chance for a goat to remain behind the initial pick is still 2/3 after the 2nd goat is revealed.

Thus you should switch to the other door, because the chance for goats are only 1/2 then.

Here's a variant to illustrate that problem. There's ten thousand doors. You pick one, monty hall opens 9998 of the other doors, revealing a farms worth of goats.
Now if you at this point don't change doors, you're a fucking moron.

>> No.2842853

>>2841428
So if we don't know Monty's intentions, do we just throw up our hands? Is there any way of giving a numerical answer?

>> No.2842886

So If the odds are 50/50 then there is no reason to switch doors.

>> No.2842907

>>2839253
0.5

>> No.2843921

bump for far more interesting than anything on page 0

>> No.2843957

IF Monty knows what's behind the doors:

At the beginning (let's say you picked door 1)
Door one has 1/3 chance
Door two has 1/3 chance
Door three has 1/3 chance

After he opens door three:
Door 1 still has 1/3 chance
Door 2 now has 1/2 chance

>> No.2843972

>>2843957
Well now that's quite a big if, isn't it?

>> No.2843981

>>2843972
Correct
Otherwise, it'd be like you picking the doors at random. The probability doesn't change if it's at random, but it does if Monty has a plan to open a door with a goat behind it

>> No.2844015

The answer is 2/3 if Monty ALWAYS opens door #2 to show a goat. If the contents of door #2 are random and he opens it anyway, it's always 50/50.

>> No.2844032

2/3

This is a well known problem. I've been "taught" this in 9th grade, 10th grade, then 12th grade (changed schools), first year of college, and then again 3rd year of college (changed school again).

At some point it ceased to be novel, if it was to begin with.

>> No.2844039

The reason why it's so hard for some people to grasp is that the numbers are so small. Make the numbers larger and it becomes a lot easier to grasp.

A million doors. You pick one. 999,998 of the doors are revealed to have goats. Are you going to switch? Absolutely.

>> No.2844044

>>2844039
But 999999 goats are worth well over a car.

>> No.2844048

>>2844044
I love you, /sci/

>> No.2844075
File: 98 KB, 435x435, 1258326935509.jpg [View same] [iqdb] [saucenao] [google]
2844075

notthisthreadagain.tiff.png.exe.bin

>> No.2844109
File: 38 KB, 500x421, 1298427296726.jpg [View same] [iqdb] [saucenao] [google]
2844109

In the original Monty Hall problem a door was opened--with prior knowledge of its contents--specifically to eliminate a bad choice which is why you should switch. If the door had been opened at random then it would have not made any difference if you switched or stayed.

To mess with you all, OP has not stated Monty's intentions and thrown in a 'smirk' to goad the reader into speculating about what he was doing.

Despite this being a blatantly fucking obvious troll thread (look at his picture) you all have descended into another shitty thread. I hereby award OP 10/10 troll points.