# Re: st: quick question about spline

 From Shige Song To statalist@hsphsun2.harvard.edu Subject Re: st: quick question about spline Date Wed, 7 Apr 2004 10:05:01 -0700 (PDT)

```Dear Roger,

That helps, thanks!

Shige

On Wed, 7 Apr 2004, Roger Newson wrote:

> At 01:02 07/04/04 -0700, Shige wrote:
> >Dear All,
> >
> >For example, in a regression model where age is one of the independent
> >variables. I am not sure the functional form of age so I want to use
> >age spline in the model. I choose cubic spline using "frencurv, power(3)".
> >Now I want to test whether there is gender difference in age effect, can I
> >simplely generate a set of interaction terms between gender and the set of
> >spline variables or there are something else I need to do? Thanks!
>
> One possibility is to define separate splines for males and females. For
> instance, if you have a basis of reference splines sp_1,
> sp_2 etc. and a variable -sex- which is one for males and two for females,
> then you can type
>
> foreach X of var sp_* {
>    gene m`X'=`X'*(sex==1)
>    gene f`X'=`X'*(sex==2)
> }
>
> and create a list of variables msp_1, msp_2 etc. which are equal to the
> spline for males and to zero for females, and a list of variables fsp_1,
> fsp_2 etc. which are equal to zero for males and to the spline for females.
> If you then type
>
> regress y msp_* fsp_*
>
> then you will have the combined set of parameters for 2 spline models, 1
> for males and 1 for females. And you can measure "interactions" (ie
> differences between differences) using -lincom-. For instance, if sp_3 is
> the reference spline at age 30 and sp_5 is the reference spline at age 50,
> then you can type
>
> lincom msp_5-msp_3-fsp_5+fsp_3
>
> and calculate a confidence interval for the difference between the male
> change between ages 30 and 50 and the female change between ages 30 and 50.
>
> I hope this helps.
>
> Roger
>
>
> --
> Roger Newson
> Lecturer in Medical Statistics
> Department of Public Health Sciences
> King's College London
> 5th Floor, Capital House
> 42 Weston Street
> London SE1 3QD
> United Kingdom
>
> Tel: 020 7848 6648 International +44 20 7848 6648
> Fax: 020 7848 6620 International +44 20 7848 6620
>    or 020 7848 6605 International +44 20 7848 6605
> Email: roger.newson@kcl.ac.uk
> Website: http://www.kcl-phs.org.uk/rogernewson
>
> Opinions expressed are those of the author, not the institution.
>
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```