[R] how to test whether two slopes are sign. different?

Liaw, Andy andy_liaw at merck.com
Mon Jul 21 14:58:36 CEST 2003

Here's the "classical" (ANCOVA?) approach, as one would learn in an applied
regression course:

summary(lm(num.book ~ pop + age + pop:age, data=your.data))

Where your.data is a data frame with at least three columns: num.book, age,
and an indicator, pop, that tells which population the subject belongs (must
be a factor).  The interaction term, pop:age, gives you a test of equal

This, of course, assume that the residual variance is the same between the
two populations.  It gets trickier if they're not, as Ripley pointed out.


> -----Original Message-----
> From: Gijsbert Stoet [mailto:stoet at volition.wustl.edu] 
> Sent: Sunday, July 20, 2003 10:51 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] how to test whether two slopes are sign. different?
> Hi,
>   suppose I do want to test whether the slopes (e.g. determined with
> lsfit) of two different population are significantly 
> different, how do I test this (in R). Say for example, I 
> found out what the slope between age and number of books read 
> per year is for two different populations of subjects (e.g. 
> 25 man and 25 woman), say using lsfit. How can I tell whether 
> the slopes are different in R. (And how would I do it for 
> regression coefficients?)
> Thanks a lot for your help.
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list 
> https://www.stat.math.ethz.ch/mailman/listinfo> /r-help

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