RE: st: RE: mkspline and reg

["stata_user stata_user" <stata_dofile@hotmail.co.uk>]

Thanks Maarten,
Your comments are useful, and graphics allow to visualize nicely the results

I think for the example that you propose it will be better to consider
gen mpg1b = mpg1 - 20

This will allow to interpret the intercept as the average value of  price 
for mpg=20 and foreign, this important because we include an interaction 
term

Regards

Sami



> From: "Maarten Buis" <M.Buis@fsw.vu.nl>
> Reply-To: statalist@hsphsun2.harvard.edu
> To: <statalist@hsphsun2.harvard.edu>
> Subject: st: RE: mkspline and reg
> Date: Wed, 1 Nov 2006 10:21:58 +0100h
>
> Sami:
> One way to see if your approach makes sense is to create predicted values, 
> graph tem and
> see if they make sense, like in the example below. I think it follows your 
> procedure and the
> graphs look fine.
>
> HTH,
> Maarten
>
> *-------------- begin example ------------
> sysuse auto, clear
> mkspline mpg1 20 mpg2=mpg
>
> gen forXmpg1 = foreign*mpg1
> gen forXmpg2 = foreign*mpg2
> reg price mpg1 mpg2 forXmpg1 forXmpg2 foreigngen mpg1b = mpg1 - 12
>
> predict pricehat1
> twoway line pricehat1 mpg if foreign==0, sort  || /*
>     */ line pricehat1 mpg if foreign==1, sort  /*
>     */ ytitle("predicted price") name(a) /*
>     */ legend(label(1 "domestic") label(2 "foreign"))
>
>
> gen forXmpg1b = foreign*mpg1b
> reg price mpg1b mpg2 forXmpg1b forXmpg2 foreign
>
> predict pricehat2
> twoway line pricehat2 mpg if foreign==0, sort  || /*
>     */ line pricehat2 mpg if foreign==1, sort  /*
>     */ ytitle("predicted price") name(b) /*
>     */ legend(label(1 "domestic") label(2 "foreign"))
> *--------------- end example ---------------
>
>
> -----------------------------------------
> Maarten L. Buis
> Department of Social Research Methodology
> Vrije Universiteit Amsterdam
> Boelelaan 1081
> 1081 HV Amsterdam
> The Netherlands
>
> visiting adress:
> Buitenveldertselaan 3 (Metropolitan), room Z434
>
> +31 20 5986715
>
> http://home.fsw.vu.nl/m.buis/
> -----------------------------------------
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu 
> [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of stata_user 
> stata_user
> Sent: dinsdag 31 oktober 2006 23:28
> To: statalist@hsphsun2.harvard.edu
> Subject: st: mkspline and reg
>
>
> I 've a variable age which takes values from 20 to 60,
> I have an outcome y and a dummy variable represnting gender
>
> I would like to fit a regresion model using linear splines, including an
> interraction effect
>
> I want to test if there are two different slopes before 40 and after 40, 
> and
> I am also interested on the intercept , I consider the following approach
>
> 1/ First create linear splines
>
> mkspline age1 40 age2 = age
>
> 2/Second  in order to have intercept at age=40
>
> generate age1a = age1 -40
>
> 3/
> I fit a regression model
>
> xi: regress y i.sex*age1a i.sex*age2
>
> Could you please comment on this approach?
>
>
>
> *
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