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Topic: [ap-stat] 30 and the Central Limit Theorem
Replies: 6   Last Post: Mar 23, 2012 7:21 PM

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 COlsen@mchsi.com Posts: 1,293 Registered: 5/17/09
RE: [ap-stat] 30 and the Central Limit Theorem
Posted: Mar 22, 2012 2:57 PM

Jared -

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So does a more nuanced explanation go like this?

There are some mathematical and historical reasons to believe that when n >
30, this is a safe point at which to believe that a sample mean is becoming
approximately normal.

However, we still like to graph our data, when possible, as an indication of
how skewed the population might be. And how confident we are that the mean
we are working with has an approximately normal distribution.

And the practice switching from t to z at 30 (or 40) is an artifact of a
lack of inadequate technology. Now that we can easily compute t for any
sample size, there is no need act as if something magically changes at 30.

Is that more complete?

Gosh, that sounds great to me! (I think the historical reason would be
that Student thought it reasonable.)

n Chris

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