# Re: st: probability question

 From Richard Goldstein To statalist@hsphsun2.harvard.edu Subject Re: st: probability question Date Wed, 28 Oct 2009 10:07:24 -0400

```I don't understand when you say "the probability that the client is
wearing a hat is 3/4."; a hat is worn on 4 out of 120 days. if you could
explain this a bit further, that would be great

vmz@vol.net.mt wrote:
> the probability that it rains on a given day is 7/120.The probability
> that it rains on three given days is [7/120]^3.The probability that the
> client is wearing a hat is 3/4.The probability that it rains on three
> given days and that the client is wearing a hat is [[7/120]^3] * 3/4  =
> 00014887,the way I see it.
>
> n 28/10/2009, "Richard Williams" <Richard.A.Williams.5@ND.edu> wrote:
>
>> At 08:22 AM 10/28/2009, Jeph Herrin wrote:
>>
>>> Prob(rain) = 7/120
>>> Prob(no rain) = 113/120
>>>
>>> X=days it rains on hat
>>>
>>> Prob(X=3)=c(4,3)*(7/120)^3*(113/120)^1
>>>           =4*0.0001985*0.94
>>>           =0.0007477
>>>
>>>
>>> not much chance!
>> Don't the probabilities change though?  e.g. if it rains on the first
>> day, then the probability of rain on the 2nd day is 6/119.
>>
>> Maybe Maarten can just run a couple million simulations for us...
>>
>>
>> -------------------------------------------
>> Richard Williams, Notre Dame Dept of Sociology
>> OFFICE: (574)631-6668, (574)631-6463
>> HOME:   (574)289-5227
>> EMAIL:  Richard.A.Williams.5@ND.Edu
>> WWW:    http://www.nd.edu/~rwilliam
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```