Date: Fri, 23 May 2008 12:16:24 -0400
Reply-To: Richard Ristow <firstname.lastname@example.org>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Richard Ristow <email@example.com>
Subject: Re: comparing groups before and after treatment
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At 05:51 PM 5/21/2008, David Hitchin wrote:
>Further to my previous suggestion, on the assumption that you have
>"before" and "after" measures for each variable, compute "change" =
>"after" - "before". A positive change is an improvement, a negative
>one a deterioration. You might then use Explore to produce a
>box-and-whisker plot, ...
Yes. I'd also present side-by-side box-and-whisker plots of 'before'
and 'after'. That will be illuminating, though for 'before' vs.
'after' designs, it may be too pessimistic. For example, suppose the
standard deviation of 'before' is 50, all patients improved 10
points, and a 10-point improvement is clinically meaningful. Then the
side-by-side plots will look like little has happened, when actually
something useful has.
>Statisticians nearly always prefer nice clear presentations close to
>the original data, ideally in plot form, rather than significance tests.
Also true, though the significance tests also MUST be performed, and
presented. As has been repeatedly said, there are many circumstances
in which a significant result on a statistical test is meaningless.
However, if the relevant statistical test fails to show significance,
it is conclusive that no effect has been observed.
(An effect may still *exist*, and may be observable otherwise, but
that's another story.)
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