You are right, but I wonder what to do if only square transformation
is recommended, for example. Should I use nonparametric tests?
Thanks,
Andreea
On Fri, Jan 15, 2010 at 6:19 PM, Lachenbruch, Peter
<[email protected]> wrote:
> I would also be very cautious in using powers greater than 1.5 in transformations since outliers can affect the behavior of such transformed variables and also affect which transformation is selected.
> This is one concern for the robustness of variance estimates when outliers are present.
>
> Tony
>
> Peter A. Lachenbruch
> Department of Public Health
> Oregon State University
> Corvallis, OR 97330
> Phone: 541-737-3832
> FAX: 541-737-4001
>
>
> -----Original Message-----
> From: [email protected] [mailto:[email protected]] On Behalf Of Carlo Lazzaro
> Sent: Friday, January 15, 2010 5:49 AM
> To: [email protected]
> Cc: 'Andreea Cristiana Didilescu'
> Subject: st: R: data transformation - need help
>
>
>
> Dear Andreea,
> the following articles might be of some interest (please, see www.bmj.com):
>
> J Martin Bland and Douglas G Altman. Statistics Notes: Transforming data.
> BMJ 1996;312;770;
>
> J Martin Bland and Douglas G Altman. Statistics notes: Transformations,
> means,and confidence intervals. BMJ 1996;312;1079
>
> J Martin Bland and Douglas G Altman. Statistics Notes: The use of
> transformation. BMJ 1996;312;1153.
>
> Kind Regards,
> Carlo
>
> -----Messaggio originale-----
> Da: [email protected]
> [mailto:[email protected]] Per conto di Andreea Cristiana
> Didilescu
> Inviato: venerdì 15 gennaio 2010 8.54
> A: [email protected]
> Oggetto: st: data transformation - need help
>
> Hello,
>
> Could anyone help me with some data transformations? I have two
> samples (n=45) with skewed distribution. After using ladder command,
> it came out that square transformation would be suitable for both of
> them. May I perform a ttest after performing data transformation? How
> should I deal with the results? Should I go back to the original data
> (e.g. mean difference)? What about the p-value?
>
> Thanks in advance,
> Andreea
>
> --
> Andreea Didilescu, DDS, PhD
> Senior Lecturer, Faculty of Dental Medicine
> "Carol Davila" UMP, Bucharest, Romania
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--
Andreea Didilescu, DDS, PhD
Senior Lecturer, Faculty of Dental Medicine
"Carol Davila" UMP, Bucharest, Romania
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