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Topic: Help with probability&stat problem
Replies: 47   Last Post: Jun 16, 2007 4:32 AM

 Messages: [ Previous | Next ]
 Bruce Weaver Posts: 753 Registered: 12/18/04
Re: Help with probability&stat problem
Posted: Jun 11, 2007 2:02 PM

tutorny wrote:
> Thanks so much for your reply!
>

>> I read that as the probability that 30 or more leave. You've apparently
>> read it as the probability that 30 or fewer leave. The longer I look
>> at the question, the less sure I am which of us is correct.

>
> A lot of us in the class had issue with the wording of the problem.
> We emailed the professor, and she said that the correct understanding
> is "30 or fewer leave" or P(x <=30), so that's what I've been working
> with.
>

>> I personally agree that the normal approximation
>> is a good fit for this kind of question. Especially since we're not
>> way out on the tail of the curve.

>
> Exactly the reasoning I used. The book said that normal approximation
> should work as long as p isn't too close to 0 or 1, and I figured that
> 0.20 should be reasonable for this.
>

>>> We want P(x <=30)
>>> When x = 30, z = (x - u)/s.d = (30 - 22)/4.1952 = 1.9069

>> Here, I think you've committed a fencepost error. If you're treating
>> a normal distribution as if it were a discrete histogram then you
>> want to put your cutoff points between the bars on the histogram, not
>> in the middle of the bars. You want to look at x=29.5 or x=30.5.
>>
>> You decide whether to use the x=29.5 or the x=30.5 cutoff by considering
>> whether the case when x=30 is included or excluded in the set of cases
>> you are looking for.

>
> I totally missed that point, and looking over the chapter I see that
> you are right. Since this is not continuous, I have to use either
> 29.5 or 30.5. Since I've confirmed that the question asks for P(x <=
> 30) I don't think that I can use 30.5, since that's > 30, so I have to
> use 29.5.

Think again. How would you work out the probability that exactly 30
left? See if your textbook has any examples like that.

>
> So, for x = 29.5: z = (x - u)/s.d = (29.5 - 22)/4.1952 = 1.7878
> P(z <= 1.7878) = 0.9631 and the answer is 96.31% which is also
> reasonable.
>
> How is that?
>
> Thanks!!
>

--
Bruce Weaver
www.angelfire.com/wv/bwhomedir

Date Subject Author
6/11/07 tutorny
6/11/07 briggs@encompasserve.org
6/11/07 tutorny
6/11/07 Randy Poe
6/11/07 tutorny
6/11/07 Bruce Weaver
6/11/07 tutorny
6/11/07 Bruce Weaver
6/11/07 C6L1V@shaw.ca
6/11/07 Randy Poe
6/11/07 tutorny
6/11/07 Nick
6/11/07 Jack Tomsky
6/11/07 Luis A. Afonso
6/11/07 Jack Tomsky
6/11/07 Luis A. Afonso
6/11/07 Jack Tomsky
6/12/07 Luis A. Afonso
6/12/07 Jack Tomsky
6/12/07 Luis A. Afonso
6/12/07 Jack Tomsky
6/12/07 Luis A. Afonso
6/13/07 Jack Tomsky
6/13/07 Luis A. Afonso
6/13/07 Anon.
6/13/07 Jack Tomsky
6/13/07 Jack Tomsky
6/13/07 Luis A. Afonso
6/13/07 Jack Tomsky
6/13/07 Anon.
6/13/07 Luis A. Afonso
6/13/07 Jack Tomsky
6/13/07 Jack Tomsky
6/13/07 Anon.
6/13/07 Luis A. Afonso
6/13/07 Jack Tomsky
6/13/07 Luis A. Afonso
6/13/07 Jack Tomsky
6/14/07 Luis A. Afonso
6/14/07 Luis A. Afonso
6/14/07 Jack Tomsky
6/14/07 Luis A. Afonso
6/15/07 Luis A. Afonso
6/15/07 Jack Tomsky
6/15/07 Luis A. Afonso
6/15/07 Jack Tomsky
6/15/07 Luis A. Afonso
6/16/07 Luis A. Afonso