[Rd] nobs() and logLik()

Berwin A Turlach Berwin.Turlach at gmail.com
Fri Jan 20 06:30:14 CET 2012

Dear all,

I am studying a bit the various support functions that exist for
extracting information from fitted model objects.

From the help files it is not completely clear to me whether the number 
returned by nobs() should be the same as the "nobs" attribute of the
object returned by logLik().  

If so, then there is a slight inconsistency in the methods for 'nls'
objects with logLik.nls() taking zero weights into account while
nobs.nls() does not.  Admittedly, the help page of nobs() states that:

	For 'lm' and 'glm' fits, observations with zero weight are not

i.e. does not comment on what nls does.  

But I wonder whether the following behaviour is desirable:

R> DNase1 <- subset(DNase, Run == 1)
R> fm3DNase2 <- nls(density ~ Asym/(1 + exp((xmid - log(conc))/scal)), 
+         data = DNase1, weights=c(0,rep(1,14),0), 
+         start = list(Asym = 3, xmid = 0, scal = 1))
R> nobs(fm3DNase2)
[1] 16
> logLik(fm3DNase2)
'log Lik.' 42.62777 (df=4)
> nobs(logLik(fm3DNase2))
[1] 14


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