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# Re: st: centred mean age

 From David Greenberg To statalist@hsphsun2.harvard.edu, T.Norris2@lboro.ac.uk Subject Re: st: centred mean age Date Wed, 30 Jan 2013 15:27:33 -0500

```Why do you regard this as a problem? David Greenberg, Sociology
Department, New York University

On Wed, Jan 30, 2013 at 2:08 PM, Thomas Norris <T.Norris2@lboro.ac.uk> wrote:
> Dear statalisters,
>
> 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.
>
> Any advice would be greatly appreciated.
>
> Many thanks,
>
> 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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```