From: Thomas Lumley <tlumley_at_u.washington.edu>

Date: Wed, 2 Jan 2008 07:58:46 -0800 (PST)

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. Received on Wed 02 Jan 2008 - 16:00:34 GMT

Date: Wed, 2 Jan 2008 07:58:46 -0800 (PST)

On Tue, 1 Jan 2008, Charles C. Berry wrote:

> On Tue, 1 Jan 2008, zhijie zhang wrote:

*>
**>> Dear all,
**>> I have two variables, y and x. It seems that the relationship between them
**>> is Piecewise Linear Functions. The cutpoint is 20. That is, when x<20, there
**>> is a linear relationship between y and x; while x>=20, there is another
**>> different linear relationship between them.
**>> How can i specify their relationships in R correctly?
**>> # glm(y~I(x<20)+I(x>=20),family = binomial, data = point) something like
**>> this?
**>
**> Try this:
**>
**>> library(splines)
**>> fit <- glm( y ~ bs( x, deg=1, knots=20 ), family=binomial)
**>
*

In the linear case I would actually argue that there is a benefit from constructing the spline basis by hand, so that you know what the coefficients mean. (For quadratic and higher order splines I agree that pre-existing code for the B-spline basis makes a lot more sense).

For example, in

fit <- glm( y ~ pmax(x,20)+pmin(x,20), family=binomial) the coefficients are the slope when is < 20 and the slope when x>20.

-thomas

Thomas Lumley Assoc. Professor, Biostatistics tlumley_at_u.washington.edu University of Washington, Seattle ______________________________________________R-help_at_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. Received on Wed 02 Jan 2008 - 16:00:34 GMT

Archive maintained by Robert King, hosted by
the discipline of
statistics at the
University of Newcastle,
Australia.

Archive generated by hypermail 2.2.0, at Wed 02 Jan 2008 - 17:30:05 GMT.

*
Mailing list information is available at https://stat.ethz.ch/mailman/listinfo/r-help.
Please read the posting
guide before posting to the list.
*