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From |
Shige Song <ssong@nicco.sscnet.ucla.edu> |

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. > > * > * For searches and help try: > * http://www.stata.com/support/faqs/res/findit.html > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**Re: st: quick question about spline***From:*Roger Newson <roger.newson@kcl.ac.uk>

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