On Mon, Mar 24, 2008 at 8:11 AM, Duncan Murdoch <[EMAIL PROTECTED]> wrote: > On 3/24/2008 9:03 AM, zhijie zhang wrote: > > Dear Murdoch, > > > Thanks very > > I would use predict() instead. What you have there doesn't look as > though it uses the B-spline basis. > > The reference given in the ?bs help page is a reasonable starting point, > but just about any book that covers splines should handle the B-spline > basis and the linear case. > > Duncan Murdoch >
Dear Duncan and others: Can you please refer us to an understandable book that explains about b-splines? I've tried a few and the math pretty quickly becomes unintelligible to a non-math major. And if this book explains the "orthogonal polynomials" that are used to represent ordinal factors in R, it would be even better. Pointing attention back to this poster's original question, I would say that using bs() here is like hitting a mosquito with a 1000 pound bomb. There are easier ways to get the estimates for the slope and intercept shifts than with bs. I'd be tempted to take the simplest approach and create a factor to represent the levels of distance, as in myDistance <- ifelse( distance > 24, "hi", ifelse(distance > 16.13, "med", "low")) ### Coerce that to a factor myDistance <- factor(myDistance) That creates a factor with three levels, and in the regression glm (mark~x+poly(elevation,2)+ distance*myDistance ...) will give the change values for the intercepts and slopes for the segments. If you want the coefficients of the 3 separate lines, run it like this instead: glm (mark~-1 + x+poly(elevation,2)+ myDistance / distance ...) Also, I'd mention that the package "segmented" is available, but it has the added feature that it will try to estimate the breakpoints from the data. PJ -- Paul E. Johnson Professor, Political Science 1541 Lilac Lane, Room 504 University of Kansas ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
