R help - Nov 2008 - mle2 simple question - sigma?

I'm trying to get started with maximum likelihood estimation with a
simple regression equivalent out of Bolker (Ecological Models and Data
in R, p302).

With this code:
#Basic example regression
library(bbmle)
RegData<-data.frame(c(0.3,0.9,0.6),c(1.7,1.1,1.5))
names(RegData)<-c("x", "y")

linregfun = function(a,b,sigma) {
  Y.pred = a+b*x
  -sum(dnorm(y,mean=Y.pred,sd=sigma,log=TRUE))
}
SigA<-mle2(linregfun, start=list(a=0.5, b=2), data=RegData)
#Or
SigB<-mle2(y~dnorm(mean=a+b*x,sd=sigma),start=list(a=0.5, b=2),
data=RegData)

I get:
#Messages
#SigA>>Error in dnorm(y, mean = Y.pred, sd = sigma, log = TRUE) :
#  element 3 is empty;
#   the part of the args list of '.Internal' being evaluated was:
#   (x, mean, sd, log)
#SigB>>Error in eval(expr, envir, enclos) : object "sigma" not
found

Obviously, I'm missing something, but what?
Thanks,
Cliff

Rice, Cliff G (DFW) wrote:
> I'm trying to get started with maximum likelihood estimation with a
> simple regression equivalent out of Bolker (Ecological Models and Data
> in R, p302).
> 
> With this code:
> #Basic example regression
> library(bbmle)
> RegData<-data.frame(c(0.3,0.9,0.6),c(1.7,1.1,1.5))
> names(RegData)<-c("x", "y")
> 
> linregfun = function(a,b,sigma) {
>   Y.pred = a+b*x
>   -sum(dnorm(y,mean=Y.pred,sd=sigma,log=TRUE))
> }
> SigA<-mle2(linregfun, start=list(a=0.5, b=2), data=RegData)
> #Or
> SigB<-mle2(y~dnorm(mean=a+b*x,sd=sigma),start=list(a=0.5, b=2),
> data=RegData)
> 
> I get:
> #Messages
> #SigA>>Error in dnorm(y, mean = Y.pred, sd = sigma, log = TRUE) :
> #  element 3 is empty;
> #   the part of the args list of '.Internal' being evaluated was:
> #   (x, mean, sd, log)
> #SigB>>Error in eval(expr, envir, enclos) : object "sigma"
not found
> 
> Obviously, I'm missing something, but what?
A starting value for sigma, I think.

(Incidentally, if you start with sigma close to its apparent MLE of 
0.047, it refuses to converge from those starting values for a and b.)


-- 
    O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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