[R] Overfitting in snr (ASSIST)

[sdfrost at UCSD.Edu]

Dear R-Help group,

I'm trying to fit some smoothing spline models using the snr function in the 
ASSIST library. My model is the following:

y~log(exp(a0-a1*x)+exp(b0-b1(x)))

where b1(x) is a nonparametric function. It's basically a biexponential decay, 
with wiggliness in 'phase 2' decay.

I can easily fit this using regression splines, but I'd like to see how 
smoothing splines work in this scenario. I tried to use the snr function, as 
follows

snr(y~log(exp(a0-a1*x)+exp(b0-b1(x))),func=list(b1(u)~list(~u,cubic2
(u))),params=list(a0+a1+b0~1),data=mydata,start=list(params=mystart))

This runs happily, but interpolates the data, rather than smoothing it. This 
is probably due to some identifiability issues, but I'm not sure how to 
parameterize the model to fix this. Any suggestions?

Best wishes
Simon