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st: centred mean age

 From Thomas Norris To "statalist@hsphsun2.harvard.edu" Subject st: centred mean age Date Wed, 30 Jan 2013 19:08:14 +0000

```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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