Re: st: quick question about spline

[Roger Newson <roger.newson@kcl.ac.uk>]

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
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United Kingdom

Tel: 020 7848 6648 International +44 20 7848 6648
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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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