R help - Sep 2003 - tree mortality risk model using glm()

Dear all,

I've used glm(family=binomial(link="logit")) several times, but
now I think
that a log link is more appropriate. 

I want to fit a model for probability of tree fall (TF)), with tree
diameter (dbh) and soil moisure (soil) as predictors. A large number of
trees have been checked every second year whether they stand up (0) or have
fallen (1).

I assume that the tree fall probability is predicted by

TF = 1 - exp(-(dbh + soil))
log(1 - TF) = -(dbh + soil)

I thought the following call would fit the model, but I get an error message.
test<- glm(1-TF ~ dbh +soil , data = extdat, family = quasibinomial(link
"log"))
Error: no valid set of coefficients has been found:please supply starting
values
In addition: Warning message: 
NaNs produced in: log(x) 

Could someone give a clue on what is wrong. Is there another way to fit
this model? 

People have asked about exponential models before but they have dealt with
continuous responses.


Thanks in advance!

Yours sincerely,
Tord



-----------------------------------------------------------------------
Tord Sn?ll
Avd. f v?xtekologi, Evolutionsbiologiskt centrum, Uppsala universitet
Dept. of Plant Ecology, Evolutionary Biology Centre, Uppsala University
Villav?gen 14			
SE-752 36 Uppsala, Sweden
Tel: 018-471 28 82 (int +46 18 471 28 82) (work)
Tel: 018-25 71 33 (int +46 18 25 71 33) (home)
Fax: 018-55 34 19 (int +46 18 55 34 19) (work)
E-mail: Tord.Snall at ebc.uu.se
Check this: http://www.vaxtbio.uu.se/resfold/snall.htm!

On Tue, 9 Sep 2003, Tord Snall wrote:
> Dear all,
>
> I've used glm(family=binomial(link="logit")) several times,
but now I think
> that a log link is more appropriate.
>
> I want to fit a model for probability of tree fall (TF)), with tree
> diameter (dbh) and soil moisure (soil) as predictors. A large number of
> trees have been checked every second year whether they stand up (0) or have
> fallen (1).
>
> I assume that the tree fall probability is predicted by
>
> TF = 1 - exp(-(dbh + soil))
> log(1 - TF) = -(dbh + soil)
>
> I thought the following call would fit the model, but I get an error
message.
> test<- glm(1-TF ~ dbh +soil , data = extdat, family = quasibinomial(link
> "log"))
> Error: no valid set of coefficients has been found:please supply starting
> values
> In addition: Warning message:
> NaNs produced in: log(x)
Although glm() usually doesn't need starting values, it can run into
problems when you have a restricted parameter space as you do here.  You
may need to supply starting values, as the error message suggests.

	-thomas