Dear R-help,
my aim is to fit a constrained 2 component gamma mixture using mix
within package mixdist. The constraints being fixing both of the means
and both of the sigmas. Thus the only unconstrained parameter I wish to
fit is pi the proportions of the mixtures.
This is the code I used:
PP2Pars<-data.frame(c(0.23,0.77), c(9.174,29.54), c(9.983,12.369));
names(PP2Pars)<-c("pi", "mu", "sigma")
mixk2<-mix(NotLinkedPP3, PP2Pars,
dist="gamma",constr=list(conpi="NONE",conmu = "MFX", consigma = "SFX",
fixpi = NULL, fixmu = c(TRUE,TRUE), fixsigma = c(TRUE,TRUE), cov =
NULL, size = NULL))
If I allow any one of the 4 constrained parameters to become
unconstrained then the model runs and gives sensible results but if I
constrain all four parameters then I get the following error.
"Error in nlm(multineg2llg, mulpseudomixdat = mspseudomixdat, mulmixpar
= msmixpar, :
invalid parameter length"
My hunch is that this is not a data-specific issue but something to do
with over-constraining the model but I can't see why and I can't find
this question having been asked before.
Please can you advise me why this is a problem and whether there is a
way of doing what I wish to do.
yours,
Roger Humphry
--
Dr Roger Humphry, Applied Statistician,
Epidemiology Research Unit, SAC, Inverness
01463 246059
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