Hi. I'm attempting to fit a logistic/binomial model so I can determine
the influence of landscape on the probability that a box gets used by a
bird. I've looked at a few sources (MASS text, Dalgaard, Fox and
google) and the examples are almost always based on tabular predictor
variables. My data, however are not. I'm not sure if that is the
source of the problems or not because the one example that includes a
continuous predictor looks to be coded exactly the same way. Looking at
the output, I get estimates for each case when I should get a single
estimate for purbank. Any suggestions?
Many thanks,
Jeff
THE DATA: (200 boxes total, used [0 if unoccupied, 1 occupied], the rest
are landscape variables).
box use purbank purban2 purban1 pgrassk pgrass2 pgrass1 grassdist
grasspatchk
1 1 0.003813435 0.02684564 0.06896552 0.3282487
0.6845638 0.7586207 0 3.73
2 1 0.04429451 0.1610738 0.1724138 0.1534174
0.3825503 0.6551724 0 1.023261
3 1 0.04458785 0.06040268 0 0.1628043
0.557047 0.7586207 0 0.9605769
4 1 0.06072162 0.2080537 0.06896552 0.01936052
0 0 323.1099 0.2284615
5 0 0.6080962 0.6979866 0.6896552 0.03168084
0.1275168 0.2413793 30 0.2627027
6 1 0.6060428 0.6107383 0.3448276 0.04077442
0.2885906 0.4482759 30 0.2978571
7 1 0.3807568 0.4362416 0.6896552 0.06864183
0.03355705 0 94.86833 0.468
8 0 0.3649164 0.3154362 0.4137931 0.06277501
0.1275168 0 120 0.4585714
THE CODE:
box.use<- read.csv("c:\\eabl\\2004\\use_logistic2.csv", header=TRUE)
attach(box.use)
box.use <- na.omit(box.use)
use <- factor(use, levels=0:1)
levels(use) <- c("unused", "used")
glm1 <- glm(use ~ purbank, binomial)
THE OUTPUT:
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -4.544e-16 1.414e+00 -3.21e-16 1.000
purbank0 2.157e+01 2.923e+04 0.001 0.999
purbank0.001173365 2.157e+01 2.067e+04 0.001 0.999
purbank0.001466706 2.157e+01 2.923e+04 0.001 0.999
purbank0.001760047 6.429e-16 2.000e+00 3.21e-16 1.000
purbank0.002346729 2.157e+01 2.923e+04 0.001 0.999
purbank0.003813435 2.157e+01 2.923e+04 0.001 0.999
purbank0.004106776 2.157e+01 2.067e+04 0.001 0.999
purbank0.004693458 2.157e+01 2.067e+04 0.001 0.999
****************************************
Jeffrey A. Stratford, Ph.D.
Postdoctoral Associate
331 Funchess Hall
Department of Biological Sciences
Auburn University
Auburn, AL 36849
334-329-9198
FAX 334-844-9234
http://www.auburn.edu/~stratja
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