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From | Nick Cox <njcoxstata@gmail.com> |
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. * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/