[R] Error with nls

[Shawn Hornsby]

Dear Kind,

I would like to let you know up front that I am not a mathematician nor do I
want to insult you or your intelligence. I am a humble person of simple
education and means and I am offering a suggestion, you may have already
resolved this issue. Here are my suggestions:

In the expression, db5 = - (k50+k56)*b5 + k56*b6 + c*g(t) + h, I notice that
function g(t) is not explicitly defined as an expression. I do see you make
reference to it in, assuming that g(t) has the functional form: b4i +
(b40-b4i)*exp(-k4*t), maybe this is the expression and I am overlooking it.
While, I continued to examine the function g(t), I saw no explicit
definition of variables b40, b4i, k4, and t. They are shown to be part of
the equation but they are not explicitly defined with values.

Again, I am hoping my observation spark an idea that would lead you to your
resolve.

If there is someone in the R-Project that has already helped Kind, I would
like to thank you.

Regards,
Shawn

-----Original Message-----
From: owner-r-help at stat.math.ethz.ch
[mailto:owner-r-help at stat.math.ethz.ch]On Behalf Of
1-27206531-0-90000491
Sent: Wednesday, March 27, 2002 4:48 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Error with nls


 Dear R-group members,

 I use:

 platform i386-pc-mingw32
 arch     x86
 os       Win32
 system   x86, Win32
 status
 major    1
 minor    4.1
 year     2002
 month    01
 day      30
 language R

 I try to fit a 2 compartment model. The compartments are open, connected
 to each other and are filled via constant input and a time depended
 function as well. Data describes increasing of Apo B after dialysis. Aim
 of the analysis is to test the hypothesis whether the data could described
 by two simple disconnected one compartment modes ore the "saturated
 model" holds? The first order differential equation for the saturated
 model:

 db5 = - (k50+k56)*b5 + k56*b6 + c*g(t) + h
 db6 = + k65*b5 - (k60+k65)*b6 + d

 db5, db6 are the first derivatives, b5, b6 are the functions to be
 fitted. The remaining parameters are unknown and should follow from the
 fit.

 assuming that g(t) has the functional form: b4i + (b40-b4i)*exp(-k4*t)

 (after calculations of 2 papers of A4) follows the solution:

 L5L6 <- function(b40, b4i, k4, t, p50, p56, p60, p65, pc, ph, pd, pb50,
 pb60) {

 	k50 <- exp(p50)
 	k56 <- exp(p56)
 	k60 <- exp(p60)
 	k65 <- exp(p65)
 	c   <- exp(pc)
 	h   <- exp(ph)
 	d   <- exp(pd)
 	b50 <- exp(pb50)
 	b60 <- exp(pb60)
 	a <- (k50+k56)
 	b <- k65
 	e <- k56
 	f <- (k60+k65)
 	z1 <- (-(a+f)/2 - sqrt((a+f)^2/4 - a*f + b*e))
 	z2 <- (-(a+f)/2 + sqrt((a+f)^2/4 - a*f + b*e))
 	K  <- ((z1+a)/(z2-z1))
 	B1 <- (b/(z2-z1)*b60 - K*b50)
 	A1 <- (b50-B1)
 	X1 <- (b*d/(z2-z1)-K*(c*b4i+h))
 	X2 <- (K*c*(b4i-b40))
 	X3 <- (c*b4i + h - X1)
 	X4 <- (c*(b40-b4i)- X2)
 	C1E <- (X3/(-z1)*(1-exp(z1*t)) +
 X4/(-(k4+z1))*(exp(-k4*t)-exp(z1*t)))
 	C2E <- (X1/(-z2)*(1-exp(z2*t)) +
 X2/(-(k4+z2))*(exp(-k4*t)-exp(z2*t)))
 	b5 <- (A1*exp(z1*t) + B1*exp(z2*t) + C1E + C2E)
 	b6 <- ((z1+a)/b * A1*exp(z1*t) + (z2+a)/b * B1*exp(z2*t) +
 (z1+a)/b * C1E + (z2+a)/b * C2E)
 	y <- f5*b5 + f6*b6
 	return(y)
 }

 I am in the lucky circumstances having starting values, because a nlr-fit
 succeeds, the graphical presentation of the fits looks quite nice. The nlr
 function is part of Lindsey's library(gnlm), but now I would like to apply
 Pinheiro and Bates library(nlme) and I have got an error:

> m2 <- nls(y ~ L5L6(b40, b4i, k4, t, p50, p56, p60, p65, pc, ph, pd,
> pb50, pb60),
> + data=help, start=c(p50=0.008678954, p56=-0.595153967,
> + p60=-4.602990518, p65=-0.625732096,
> + pc=-0.128657978, ph=0.708033556, pd=1.140357461, pb50=1.311141424,
> + pb60=1.270852258))
> Error in numericDeriv(form[[3]], names(ind), env) :
>         Missing value or an Infinity produced when evaluating the model
>
 If somebody feel that he can help me, I could send him my R- code and
 data file as well.

 Kind regards,

 Dominik


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