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

 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

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 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