I hadn't checked this, I only had used the default.
The problem stems from the reassigned weighs during nls iterations,
where the psi.bisquare function can return some 0 weight values, which
then returns some NaN values in the resid vector that is calculated by
dividing residuals/weights.
Since the default na.action value is na.fail, this causes the nls to crash.
you can bypass this problem with
model3=nlrob(y~a1*x^a2,data=transient,psi=psi.bisquare,
start=list(a1=0.02,a2=0.7), maxit=1000, na.action = na.omit)
but it will give you a lot of warning messages about a division of a
different size vectors in -residuals(out)/sqrt(w), but will finish and
calculate the parameters.
I don't know enough statistics to assure you that omiting values with
weight=0 is sound from a mathematical standpoint, intuitively I would
substitute them for 0.
Perhaps someone out there can help us with this?
Best wishes,
Keo.
Xiao Xiao escribió:
Thank you Keo!
After installing MASS the default "psi=psi.huber" is working now.
However I still can't get "psi=psi.bisquare" to work, and here's
another error message:
model3=nlrob(y~a1*x^a2,data=transient,psi=psi.bisquare,
start=list(a1=0.02,a2=0.7),maxit=1000)
Error in na.fail.default(list(y = c(71.2600034232749, 148.175742933206, :
missing values in object
I don't know why there are missing values, I'm sure y is the right
length and there are no NAs in it. Could somebody help me with this
one please?
Thanks in advance,
Xiao
On Wed, Aug 12, 2009 at 11:47 PM, Keo
Ormsby<christopher.orm...@cieni.org.mx> wrote:
You have to install MASS package first.
Hope this does the trick.
Best,
Keo.
Xiao Xiao wrote:
Hi all,
I'm trying to fit a nonlinear regression by "nlrob":
model3=nlrob(y~a1*x^a2,data=transient,psi=psi.bisquare,
start=list(a1=0.02,a2=0.7),maxit=1000)
However an error message keeps popping up saying that the function
psi.bisquare doesn't exist.
I also tried psi.huber, which is supposed to be the default for nlrob:
model3=nlrob(y~a1*x^a2,data=transient,psi=psi.huber,
start=list(a1=0.02,a2=0.7),maxit=1000)
But I still got the same error message - psi.huber doesn't exist.
Is the argument "psi" not available in nlrob?
Any help will be appreciated.
Best,
Xiao Xiao
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