Date: Thu, 24 Mar 2011 15:23:22 -0400
Reply-To: Mike Palij <mp26@nyu.edu>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Mike Palij <mp26@nyu.edu>
Subject: Re: about Bonferroni adjustments
Content-Type: text/plain; charset="iso-8859-1"
Hi Bridgette,
The Bonferroni procedure is technically a "planned comparison"
procedure, that is, you are only interested in certain comparisons
between means and will ignore other comparisons. To maintain
the overall alpha or Type I error = 0.05, divide 0.05 by the
number of tests you intend to do. However, if other comparisons
become of interest then you will have to fudge the alpha levels --
which means you should be pretty damn sure that there are only
certain differences that you want to look at. On the graphpad
website, there is a little write-up on the Bonferroni correction
and how it should be used. See:
http://www.graphpad.com/faq/viewfaq.cfm?faq=1092
-Mike Palij
New York University
mp26@nyu.edu
----- Original Message -----
From: "Bridgette Portman" <bportman@uci.edu>
To: <SPSSX-L@LISTSERV.UGA.EDU>
Sent: Thursday, March 24, 2011 2:50 PM
Subject: about Bonferroni adjustments
> 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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