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Topic: Help with a school project, a statistical survey on education
Replies: 15   Last Post: Jul 13, 2013 4:14 PM

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Richard Ulrich

Posts: 2,862
Registered: 12/13/04
Re: Help with a school project, a statistical survey on education
Posted: Jul 22, 2007 11:36 PM
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On Sun, 22 Jul 2007 18:36:50 -0000, loom91 <loom91@gmail.com> wrote:

[snip, some previous and replies. Just to reply to one
point]
>
> We plan on taking 50 data points from each school, about 20-25% of the
> class size. So you say that we should take the same number of points
> even if one school has more students than the other? Also, do you mean
> that the sample size is insufficient to draw reliable conclusions
> about whether coed schools really lessen the gender differences?
>


The earlier poster tried to make this point, and I will
try again. Whether you are comparing 50 or 500
students from two schools, you are only comparing
*two* schools. Or six schools. Within the schools,
there is a lesser hierarchy, perhaps, of a dozen
teachers -- or whatever. If you see differences between
two schools, is it something idiosyncratic to those
two schools, or idiosyncratic to that selection of a
dozen teachers?

If you want draw an inference about "schools", it is
almost necessary to have a large number of *schools*
represented. There is a statistical approach that tries to
test across multiple strata, but that quickly gets complicated.

With two schools, or a few schools, you can easily
do comparisons, by t-tests or ANOVA, that tell you whether
these particular schools seem to "differ". That can be a
starting point for discussing all the *wrong* reasons that
might exist that could account for the differences.
Selective admissions? Selective attrition? Particular
teachers?

Even when there are 50 or 500 *schools* being sampled,
the resulting "inference" (whether coed schools differ) can
be challenged for systematic biases. With a large number
of schools, it begins to be possible to argue that various
factors have been measured, and that they indeed do seem
to be balanced or equivalent between schools, or otherwise
accounted for.

With a few schools -- try to keep the questions simple, and
try to keep the conclusions well-tempered by doubts.

I'm sorry I can't be of more help to the questions that you
pose.

--
Rich Ulrich, wpilib@pitt.edu
http://www.pitt.edu/~wpilib/index.html



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