Date:         Thu, 24 Mar 2011 11:50:18 -0700
Reply-To:     Bridgette Portman <bportman@uci.edu>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Bridgette Portman <bportman@uci.edu>
Subject:      about Bonferroni adjustments
In-Reply-To:  <005801cbe90f$a4a948d0$edfbda70$@lange@adelaide.edu.au>
Content-Type: text/plain;charset=iso-8859-1

Hi everyone,

I have a question about Bonferroni adjustments that I'm not quite clear
on. I'm using this to do post-hoc comparisons after performing an ANOVA or
larger than 2x2 chi-square test. I know that Bonferroni adjustments
involve dividing the alpha level (.05) by the number of post-hoc
comparisons I intend to make, to get a new acceptable alpha level. But
does that mean the total number of comparisons one could make, or the
number I actually make?

Here's an example for illustration.  Suppose I have:

Group 1 mean: 45.88
Group 2 mean: 67.29
Group 3 mean: 48.24
Group 4 mean: 44.90

Let's say running ANOVA reveals significant differences. In performing
post-hoc tests, I would really only be interested in comparing Group 2
with each of the others. I can tell by looking at the means that Group 1,
Group 3 and Group 4 are not going to differ significantly. SPSS would do
Bonferroni adjustments in this case assuming that I want to compare ALL
means, i.e. make 6 comparisons. So it would set the corrected alpha level
at .05/6 or .0083. But if I only want to make 3 comparisons, then it would
be .05/3 or .017. I'm not sure which is proper.

Bridgette

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