# [R] Specify a correct formula in R for Piecewise Linear Functions?

Charles C. Berry cberry at tajo.ucsd.edu
Tue Jan 1 19:43:47 CET 2008

```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)

HTH,

Chuck

> Thanks a lot.
>
> --
> With Kind Regards,
>
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> Zhi Jie,Zhang ,PHD
> Tel:+86-21-54237149
> Dept. of Epidemiology,School of Public Health,Fudan University
> Postcode:200032
> Email:epistat at gmail.com
> Website: www.statABC.com
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