At 12:36 23/10/2009, Tord Snäll wrote:
Dear all,
A question related to the following has been
asked on R-help before, but I could not find any
answer to it. Input will be much appreciated.
The vignette explains this, and much more. I
found it extremely instructive both for its
intended purpose, count data, and for general
tips about manipulating several models and comparing them.
I got an unexpected sign of the "slope"
parameter associated with a covariate (diam)
using zeroinfl(). It led me to compare the
estimates given by zeroinfl() and hurdle():
The (significant) negative estimate here is
surprising, given the biology of the species:
> summary(zeroinfl(bnl ~ 1| diam, dist =
"poisson", data = valdaekar, EM = TRUE))
Count model coefficients (poisson with log link):
Estimate Std. Error z value Pr(>|z|) (Intercept) 3.74604
0.02635 142.2 <2e-16 ***
Zero-inflation model coefficients (binomial with logit link):
Estimate Std. Error z value Pr(>|z|) (Intercept) 21.7510
7.6525 2.842 0.00448 **
diam -1.1437 0.3941 -2.902 0.00371 **
Number of iterations in BFGS optimization: 1
Log-likelihood: -582.8 on 3 Df
The hurdle model gives the same estimates, but
with opposite (and expected) signs of the parameters:
summary(hurdle(bnl ~ 1| diam, dist = "poisson", data = valdaekar))
Count model coefficients (truncated poisson with log link):
Estimate Std. Error z value Pr(>|z|) (Intercept) 3.74604
0.02635 142.2 <2e-16 ***
Zero hurdle model coefficients (binomial with logit link):
Estimate Std. Error z value Pr(>|z|) (Intercept) -21.7510
7.6525 -2.842 0.00448 **
diam 1.1437 0.3941 2.902 0.00371 **
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Number of iterations in BFGS optimization: 8
Log-likelihood: -582.8 on 3 Df
Why is this so?
thanks,
Tord
Windows NT, R 2.8.1, pcsl 1.03
--
Tord Snäll
Department of Ecology / Swedish Species Information Centre
Swedish University of Agricultural Sciences (SLU)
P.O. 7044, SE-750 07 Uppsala, Sweden
Office/Mobile/Fax
+46-18-672612/+46-76-7662612/+46-18-673537
www.ekol.slu.se/staff_tordsnall
www.artdata.slu.se/personal/fototsn.asp
Michael Dewey
http://www.aghmed.fsnet.co.uk
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
