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 ]
Randy Poe

Posts: 5,658
Registered: 12/6/04
Re: Help with probability&stat problem
Posted: Jun 11, 2007 1:06 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

On Jun 11, 12:27 pm, tutorny <tuto...@gmail.com> wrote:
> I am looking at solving the following problem:
>
> Past records show that at a given college 20% of the students who
> began as psychology majors either changed their major or dropped out
> the school. An incoming class has 110
> beginning psychology majors. What is the probability that as many as
> 30 of these students leave the psychology program?
>
> I think that I can solve it using the normal approximation to the
> binomial probability distribution, as follows:
>
> n =110, p = 0.20
> mean = u = np = 110*0.20 = 22
> standard deviation = s.d. = (n*p*q)^.5 = (110*.20*.80)^.5 = 4.1952
>
> We want P(x <=30)
>
> When x = 30, z = (x - u)/s.d = (30 - 22)/4.1952 = 1.9069
>
> So, we want P(z<=1.9069) = 0.9717
>
> So, the answer is 97.17 percent.
>
> Can you tell me if you see anything wrong with this answer, or the way
> that I solved this problem? Any help is greatly appreciated.


Mainly that you answered the wrong question.

You are told that on average 20% drop out. So a 20% drop
out rate is most likely. A rate of 15% or 25% would be less
likely. A rate of 10% or 30% even less likely.

Do you really think there is a 97% chance that
there will be a 30% drop out rate? Suppose I asked
you the probability that 100 of the students would
drop out. How would you calculate that? Do you think
that is a higher probability than only 30 students
dropping out?

- Randy



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.