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st: ln transform and box cox


From   Thomas Norris <T.Norris2@lboro.ac.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: ln transform and box cox
Date   Wed, 6 Mar 2013 15:07:10 +0000

Dear statalisters,

I am running estimated fetal weight growth models. Weight shows increasing variance as gestation proceeds so I was advised to take the natural logarithm of weight to address this issue. However I am having doubts about whether this transformation is appropriate. Should I instead use a box cox transformation of the weight variable?

If this is the case, how would I proceed with running the box cox? I have used the help and see that the syntax is straightforward, but I don't know how I would include the independent variables (age), as in my case, this is what I am trying to find out, ie which age term(s) best describe the data using the fracpoly command. With natural log weight, the best 2 degree fracpoly was with age terms 1 and 2 (fracpoly command didn't converge on raw scale), but if I put these in the box cox, they are based on natural logged data, which is the thing I am doubting. 

Any help would be greatly appreciated,

Tom 

Tom Norris (PhD student)
Centre of Global Health and Human Development
School of Sport, Exercise and Health Sciences
Loughborough University
Loughborough
LE11 3TU


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