[R] custom selfStart model works with getInitial but not nls

[Michael A. Gilchrist]

Hello,

I'm having problems creating and using a selfStart model with nlme.  Briefly, 
I've defined the model, a selfStart object, and then combined them to make a 
selfStart.default model.

If I apply getInitial to the selfStart model, I get results.  However, if I try 
usint it with nls or nlsList, these routines complain about a lack of initial 
conditions.

If someone could point out what I'm doing wrong, I'd greatly appreciate it.


Details:
## Nonlinear model I want to fit to the data
const.PBMC.tcell.model <- function(B0, t, aL, aN, T0){

   Tb0 = B0;

   x = exp(-log(aL) + log(T0*aL+(-1+exp(t * aL))*Tb0 * aN) - t * aL);

   return(x);
}


##Define selfStart routine
const.PBMC.tcell.selfStart<- function(mCall, LHS, data){

   t0 = 0;
   t1 = 24;
   t2 = 48;

   ##Get B0 Value
   B0 =  data[1, "B0"];

   T0 = mean(data[data$Time==t0, "Count"]);
   T1 = mean(data[data$Time==t1, "Count"]);
   T2 = mean(data[data$Time==t2, "Count"]);

   if(T0 < T2){ ##increase -- doesn't work
     stop(paste("Error in const.PBMC.tcell.start: T0 < T2 for data: ", data[1, 
]));

   }
   ##Estimate aL based on exponential decline from t=0 to t=24
   aLVal = -(log(T1) - log(T0))/(t1-t0);

   ##Estimate aNVal based on final value
   aNVal = aLVal*T2/B0;

   values = list(aLVal, aNVal, T0);
   names(values) <- mCall[c("aL", "aN", "T0")]; #mimic syntax used by P&B
   return(values)
}



##Now create new model with selfStart attributes
const.PBMC.tcell.modelSS<-  selfStart(model = const.PBMC.tcell.model, 
initial=const.PBMC.tcell.selfStart)


##Test routines using getInitial -- This works
> getInitial(Count ~ const.PBMC.tcell.modelSS(B0, Time,aL, aN, T0), data = 
> tissueData)
[1] 0.05720924
$aL
[1] 0.05720924

$aN
[1] 0.1981895

$T0
[1] 1360.292

##Now try to use the SS model -- this doesn't work
> nls(Count ~ const.PBMC.tcell.modelSS(B0, Time,aL, aN, T0), data = tissueData)
Error in numericDeriv(form[[3L]], names(ind), env) :
   Missing value or an infinity produced when evaluating the model
In addition: Warning message:
In nls(Count ~ const.PBMC.tcell.modelSS(B0, Time, aL, aN, T0), data = 
tissueData) :
   No starting values specified for some parameters.
Intializing 'aL', 'aN', 'T0' to '1.'.
Consider specifying 'start' or using a selfStart model