The Math Forum



Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.


Math Forum » Discussions » sci.math.* » sci.stat.math

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

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
tutorny

Posts: 39
Registered: 1/28/06
Re: Help with probability&stat problem
Posted: Jun 11, 2007 1:27 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

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.

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!!



Date Subject Author
6/11/07
Read Help with probability&stat problem
tutorny
6/11/07
Read Re: Help with probability&stat problem
briggs@encompasserve.org
6/11/07
Read Re: Help with probability&stat problem
tutorny
6/11/07
Read Re: Help with probability&stat problem
Randy Poe
6/11/07
Read Re: Help with probability&stat problem
tutorny
6/11/07
Read Re: Help with probability&stat problem
Bruce Weaver
6/11/07
Read Re: Help with probability&stat problem
tutorny
6/11/07
Read Re: Help with probability&stat problem
Bruce Weaver
6/11/07
Read Re: Help with probability&stat problem
C6L1V@shaw.ca
6/11/07
Read Re: Help with probability&stat problem
Randy Poe
6/11/07
Read Re: Help with probability&stat problem
tutorny
6/11/07
Read Re: Help with probability&stat problem
Nick
6/11/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/11/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/11/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/11/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/11/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/12/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/12/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/12/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/12/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/12/07
Read Binomial law DF: a simple program
Luis A. Afonso
6/13/07
Read Re: Binomial law DF: a simple program
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/13/07
Read Re: Help with probability&stat problem
Anon.
6/13/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/13/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Anon.
6/13/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/13/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Anon.
6/13/07
Read FALSIFICATION
Luis A. Afonso
6/13/07
Read Re: FALSIFICATION
Jack Tomsky
6/13/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/13/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/14/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/14/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/14/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/14/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/15/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/15/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/15/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/15/07
Read Re: Help with probability&stat problem
Jack Tomsky
6/15/07
Read Re: Help with probability&stat problem
Luis A. Afonso
6/16/07
Read Re: Help with probability&stat problem
Luis A. Afonso

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2017. All Rights Reserved.