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Date:         Sun, 11 Sep 2005 15:30:23 -0300
Reply-To:     Hector Maletta <hmaletta@fibertel.com.ar>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Hector Maletta <hmaletta@fibertel.com.ar>
Subject:      Re: data transformation bibliografical sources
Comments: To: Jorge Camacho <jcamacho@ice.co.cr>
In-Reply-To:  <43246334.1040106@ice.co.cr>
Content-Type: text/plain; charset="iso-8859-1"

Jorge, Normality and homogeneous variance are possible attributes of your data, and they may or may not have them. No data transformation by itself will give them what they do not have.

You can of course transform your variables into something else that is more similar to what you desire (e.g. the logarithm of a variable may have a distribution that looks more "normal" than the original variable), and there is always the possibility of finding a mathematical formula, however abstruse, able to achieve that. But on scientific terms this would be meaningless unless you have a theory whereby your variable behaves in ways related to that particular mathematical function. For instance, if people react more to the PROPORTION their incomes grow, than the AMOUNT of the increase, and thus an additional $1000 means different things to a billionnaire or to you and me, then the logarithm of income may find a place in your analysis, because a certain difference in logarithms means a certain proportional difference in the original variable. If you do not have theory or evidence of this kind, using logarithms has as much sense as using, say, the cosine or the cubic root or a 17th degree polynomial of your variable.

Besides, remember previous caveats in this forum to the effect that it is not variables, but errors of estimation, that have to be normal, with homogeneous variances, for standard statistical models (like regression) to apply.

Hector

> -----Original Message----- > From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] > On Behalf Of Jorge Camacho > Sent: Sunday, September 11, 2005 2:03 PM > To: SPSSX-L@LISTSERV.UGA.EDU > Subject: data transformation bibliografical sources > > Dear All: > > I am loking for a good review or bibliografical source (in > electronic format if possible) about data transformation in > order to reach normallity, homogeneous variances etc. Most > text books have very few pages on this. I would appreciate > any supportt on this. > > Thanks in davance. > > Jorge > > -- > @@@@@@@@@@@@@@@@@@@@@@@@@@@@ > Jorge Camacho Sandoval, Ph. D. > Bioestadística - Mejora Genética Animal > P. O. Box 1960 - 4050, Alajuela, Costa Rica Tel. (506)4410487 > Fax. (506)4400575 > e-mail: jcamacho@ice.co.cr or jorge.camacho.s@gmail.com > @@@@@@@@@@@@@@@@@@@@@@@@@@@@ > > __________ Información de NOD32 1.1213 (20050909) __________ > > Este mensaje ha sido analizado con NOD32 Antivirus System > http://www.nod32.com > >


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