Spencer,
 
MARS fits splines, not disconnected lines.  Perhaps the strucchange package has facility to fit your data better.
 
Cheers,
Andy

________________________________

From: r-help-bounces@stat.math.ethz.ch on behalf of Spencer Graves
Sent: Tue 10/17/2006 11:43 PM
To: R-help; Kurt Hornik
Subject: [R] MARS help? [Broadcast]



      I'm trying to use mars{mda} to model functions that look fairly
close to a sequence of straight line segments.  Unfortunately, 'mars'
seems to totally miss the obvious places for the knots in the apparent
first order spline model, and I wonder if someone can suggest a better
way to do this.  The following example consists of a slight downward
trend followed by a jump up after t1=4, following by a more marked
downward trend after t1=5:

Dat0 <- cbind(t1=1:10,
   x=c(1, 0, 0, 90, 99, 95, 90, 87, 80, 77))
library(mda)
fit0 <- mars(Dat0[, 1, drop=FALSE], Dat0[, 2],
             penalty=.001)
plot(Dat0, type="l")
lines(Dat0[, 1], fit0$fitted.values,
      lty=2, col="red")

      Are there 'mars' options I'm missing or other software I should be
using?

      I've got thousands of traces crudely like this of different
lengths, and I want an automated way of summarizing similar traces in
terms of a fixed number of knots and associated slopes for each linear
spline segment max(0, t1-t.knot).

      Thanks,
      Spencer Graves

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