Re: st: centred mean age

[Richard Goldstein <richgold@ix.netcom.com>]

I don't know where you got that idea

the coefficient for the term raised to the highest power (e.g., the
cubic term in your example) will not change but the others will

Rich

On 1/30/13 3:32 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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