[R] Do YOU know an equation for splines (ns)?

[Ranae]

I have not been able to get "predict" (or most functions) to run well with
grouped data in nlme.  I may not have it coded right, but this is what it
looks like:

http://r.789695.n4.nabble.com/file/n4632567/spline.txt spline.txt 

library(nlme)
library(splines)

rootCN<-read.table("spline.txt", header=TRUE)
rootCN$plotF<-as.factor(rootCN$plot)

rcn10G<-groupedData(N ~ day | plotF, data=rcn10)

fit10 <- lme( N~ns(day, 3), data = rcn10G)

plot(augPred(fit10))

num<- seq(88,300, len=200)
lines(num, predict(fit10, data.frame(day=num)))

-Ranae


Does
 ?predict.ns
 not do what you want without having to explicitly manipulate the spline
basis? 

-- Bert 

On Tue, Jun 5, 2012 at 1:56 PM, Ranae <[hidden email]> wrote:

> Hi, 
> 
> I am looking at the change in N concentration in plant roots over 4 time 
> points and I have fit a spline to the data using ns and lme: 
> 
> fit10 <- lme( N~ns(day, 3), data = rcn10G) 
> 
> I may want to adjust the model a little bit, but for now, let's assume
> it's 
> good.  I get output for the fixed effects: 
> 
> Fixed: N ~ ns(day, 3) 
> (Intercept) ns(day, 3)1 ns(day, 3)2 ns(day, 3)3 
>  1.15676524  0.14509171  0.04459627  0.09334428 
> 
> and coefficients for each experimental unit in my experiment: 
> 
>   (Intercept) ns(day, 3)1 ns(day, 3)2 ns(day, 3)3 
> 24    1.050360 -0.42666159 -0.56290877 -0.10714407 
> 13    1.104464 -0.30825350 -0.53311653 -0.05558150 
> 31    1.147878 -0.14548512 -0.78673906 -0.07231781 
> 46    1.177781 -0.22278380 -0.80278177 -0.02321460 
> 15    1.144215 -0.04484519 -0.06084798  0.07633663 
> 32    1.213007  0.00741061  0.03896933  0.15325849 
> 23    1.274615  0.16477514  0.00872224  0.23128320 
> 41    1.215626  0.57050767  0.11415467  0.10608867 
> 43    1.134203  0.48070741  0.72112899  0.18108193 
> 12    1.091422  0.39563632  1.01521528  0.22597459 
> 21    1.100631  0.44589314  0.98526322  0.23535739 
> 35    1.226980  0.82419937  0.39809568  0.16900841 
> 
> NOW, I want to write a spline function where I can incorporate these 
> coefficients to get the predicted N concentration value for each day. 
> However, I am having trouble finding the right spline equation, since
> there 
> are many forms on the internets. 
> 
> I know it won't be a simple one, but can some one direct me to the
> equation 
> that would be best to use for ns? 
> 
> Thanks a lot, 
> 
> Ranae 

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