Questions about piecewise spline fitting

Hi,
What you need is just the bs() or ns() functions in the library splines in
the R-software. You have to specify the knots and can specify the degree.
For instance:

library(splines)
bs(1:20, knots=c(5,11), degree=1) #performs three #straight lines with
break-point at x=5 and 11 for the variable 1:20
lm(y~bs(1:20, knots=c(5,11), degree=1)) #piecewise linear regression

Common "problem" is that the beta parameters for the pseudo-variables
inducted by bs() are not interpretable (i.e. in linear case they don't
represents the slopes in each piece).
best,
vito


----- Original Message -----
From: "Gang Liang" <gumpleon at hotmail.com>
To: <r-help at hypatia.math.ethz.ch>
Sent: Wednesday, December 05, 2001 7:05 AM
Subject: [R] Questions about piecewise spline fitting

Hi All,

I want to fit a piecewise spline of degree 1, i.e. a spline consisting of

a

straight line over each piece. I downloaded the R package pspline, then I
have following questions:

1. in the program, the degree of the spline is specified by 2*norder-1.

Why

do they adopt such scheme that we can only fit a spline with odd degree?

2. norder cannot be set to 1. Is there any specific reason for doing so?

Maybe I need to code this by myself. So I'd like to hear your advices.

Thanks in advance,
Gang

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