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# Re: st: centred mean age

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: centred mean age Date Wed, 30 Jan 2013 20:37:51 +0000

Please show us what is troubling you. (See the Statalist FAQ for
advice on giving detailed, precise information.)

Nick

On Wed, Jan 30, 2013 at 8:32 PM,  <t.norris2@lboro.ac.uk> wrote:
> Hi Nick,
>
> Thank you for you reply.
>
> I was under the impression that the age coefficients in a centred model shouldn't be different to an uncentred model though, and mine change.
>
> Is this change therefore ok?
>
> Thank you,
>
> Tom
>
>
> Sent using BlackBerry® from Orange
>
> -----Original Message-----
> From: Nick Cox <njcoxstata@gmail.com>
> Sender: <owner-statalist@hsphsun2.harvard.edu>
> Date: Wed, 30 Jan 2013 20:20:48
> To: <statalist@hsphsun2.harvard.edu>
> Reply-To: <statalist@hsphsun2.harvard.edu>
> Subject: Re: st: centred mean age
>
> 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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