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Re: st: general statistical reasoning question in biomedical statistics(no Stata content)


From   cryan@binghamton.edu
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: general statistical reasoning question in biomedical statistics(no Stata content)
Date   Thu, 11 Dec 2003 14:47:25 -0500 (EST)

A very good point.  Thank you for reminding me.

> Let me be picky with just one point you make because I see it
> repeated too often, and that is the issue that, "And for every 20
> baseline variables compared, you'd *expect* about 1 of those baseline
> variables to have a P of < 0.05"
>
> This is a misquote of a mathematical tautology that says that 5% of
> all tests (1 in 20) will fall into the 5% region.  The proper quote
> is that this refers to *independent* tests.  This oversight is
> especially important here because if ever one should question the
> independence assumption it is in this situation.  When we are looking
> at a number of baseline characteristrics on the patients, it is
> probably more than likely that there is some dependence amongst them.
> For example, if the two arms are not balanced with respect to height
> with one arm getting the shorter patients, then more than likely
> that arm will have the lighter patients too.
>
> So when we do a number of related tests, we may get more or less
> than 5% significant due to chance.  It all depends on the
> structure of the dependency, a point that should be made to students.
>
> m.p.
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