On Sun, 22 Jul 2007 18:36:50 -0000, loom91 <firstname.lastname@example.org> 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.