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Re: st: centred mean age


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: centred mean age
Date   Wed, 30 Jan 2013 20:20:48 +0000

Whether or not it helps in your model, I see no problem in what you
describe. It's the way that linear, quadratic and cubic terms work
together in a model that's important.

All that said, there are quite possibly better ways of doing what you
want, such as cubic splines or fractional polynomials, which are well
supported in Stata.

Nick

On Wed, Jan 30, 2013 at 7:08 PM, Thomas Norris <T.Norris2@lboro.ac.uk> wrote:

> I am having trouble with centering my independent variable (age) in a cubic polynomial.
>
> I have generated the centred age by using gen centrage= age-r(mean) and then to get the centred quadratic and cubic I simple raise centrage to ^2 and ^3 respectively (gen centrage2= centrage^2)(gen gentrage3=centrage^3)
>
> However, the negative centred age terms (ie those smaller than the mean) become positive when squaring them, which is what is mathematically correct, but it doesn't help my models.
>
> If for example the mean was 30 weeks and I had 2 separate obs, one at 25 weeks and one at 35 weeks, the centred age would be -5 and 5, but the centred age^2 are both 25.
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