[R] How to define degree=1 in mgcv

[Gavin Simpson]

On Sun, 2010-01-24 at 16:28 -0500, hellen lee wrote:
> Hi, all
> 
> I have a question on mgcv and ns. Now I want to compare the results from
> glm, gam and ns. Take a simple model y~x for example.
> 
> glm1 = glm(y~x, data=data1)
> gam1 = gam(y~s(x), data=data1)
> ns1 = glm(y~ns(x),data=data1)
> 
> In order to confirm the result from glm1 is consistent to those from gam1
> and ns1, I want to define degree=1 in mgcv and ns. I am wondering if there
> is somebody can give me some suggestions? I appreciate.

require(mgcv)
require(splines)
data(airquality)

m1 <- glm(Ozone ~ Wind, data = airquality)
m2 <- gam(Ozone ~ Wind, data = airquality)
m3 <- glm(Ozone ~ ns(Wind, df = 1), data = airquality)

For gam, I don't think you can set a smooth to be linear, which is what
I interpret degree = 1 to be (I also don't know where you get degree = 1
from, this isn't anything to do with ns() in package splines). So we
just don't specify that Wind be a smoother in the GAM --- not sure if
that is what you wanted?

Compare the fits using say summary:

e.g. summary(m1); summary(m2)

Also, from the above, note that these are all linear models (as opposed
to generalised) and would be fitted more efficiently using lm():

m4 <- lm(Ozone ~ Wind, data = airquality)
m5 <- lm(Ozone ~ ns(Wind, df = 1), data = airquality)

> all.equal(fitted(m1), fitted(m2))
[1] TRUE
> all.equal(fitted(m1), fitted(m3))
[1] TRUE
> all.equal(fitted(m1), fitted(m4))
[1] TRUE
> all.equal(fitted(m1), fitted(m5))
[1] TRUE

HTH

G

> 
> Hellen
> 
> 	[[alternative HTML version deleted]]
> 
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