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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