There is no reason to assume that the 1000 novices are not a random sample. If you were sampling something like income or political opinion or longevity, it would make sense to look at it more carefully, as where you live can be correlated with these things. However, I don't see any plausible way that channeling strength among people who don't yet know they can channel could in any way be correlated with things like where you live or whether you would like to become Aes Sedai.

And with the potential channeling population being that large, it is perfectly reasonable to assume that each of the novices has a 62.5% chance of being strong enough to become Aes Sedai. And if this is the case, it is flat out impossible that only 200 out of 1000 are strong enough.

And what do you mean by Northern Altaran women being generally weak in the power? Didn't you just yesterday say that strength in channeling isn't genetic, and that it is unreasonable that genetics could cause a general drop in channeling strength since the Age of Legends?