R help - May 2012 - Breakpoint in logistic GLM with 'segmented' package - error: replacement length zero

Hello all,

I've been having trouble with assessing a breakpoint in a logistic GLM
with two explanatory variables. For this analysis I've been using the
'segmented' package version 0.2-9.1. But I keep getting an error and I
don't see where I would be going awry. The situation is the following:

Two explanatory variables:
bedekking - a variable with possible values between 0 and 1 - mine
runs from 0.05 to 0.5, increasing with steps of 0.05
s.size - a count variable - increases from 3 to 25 with steps of 1,
and from 25 to 60 with steps of 5

Each combination of s.size and bedekking has 100 repeats so the
resulting dataframe 'dat.al2' consists of 30000 observations of 3
variables.

Because the response variable has values between 0 and 1, I used a logistic GLM:
> gmodel <- glm(R.AUC ~ bedekking + s.size, data=dat.al2, family =
quasibinomial(link=logit))
R.AUC increases with increasing s.size and decreasing bedekking,
looking at the graph shows that the association reaches a plateau at a
s.size of 10 and a bedekking of 0.45. So these are the values I use in
'psi' argument in the 'segmented' function
> psi.mod <- list(0.45, 10)
> names(psi.mod) <- c("bedekking", "s.size")
Then I attempt to run the 'segmented' function:
> seg.gm <- segmented(obj = gmodel, seg.Z= ~bedekking + s.size, psi =
psi.mod)
When I run this, after half a minute I get the following error:

Error in ifelse(is.list(o0), o0$dev.no.gap, 10^12) :
  replacement has length zero

Does anybody know what might be causing this error, and could somebody
point out where I might go wrong?

Thanks in advance,

Peter

dear Peter,
Your code appears correct, so it is difficult to reply without the data..

If you are interested in further details, please contact me off-list

vito


Il 25/05/2012 15.34, Peter Hoitinga ha scritto:
> Hello all,
>
> I've been having trouble with assessing a breakpoint in a logistic GLM
> with two explanatory variables. For this analysis I've been using the
> 'segmented' package version 0.2-9.1. But I keep getting an error
and I
> don't see where I would be going awry. The situation is the following:
>
> Two explanatory variables:
> bedekking - a variable with possible values between 0 and 1 - mine
> runs from 0.05 to 0.5, increasing with steps of 0.05
> s.size - a count variable - increases from 3 to 25 with steps of 1,
> and from 25 to 60 with steps of 5
>
> Each combination of s.size and bedekking has 100 repeats so the
> resulting dataframe 'dat.al2' consists of 30000 observations of 3
> variables.
>
> Because the response variable has values between 0 and 1, I used a logistic
GLM:
>
>> gmodel<- glm(R.AUC ~ bedekking + s.size, data=dat.al2, family =
quasibinomial(link=logit))
>
> R.AUC increases with increasing s.size and decreasing bedekking,
> looking at the graph shows that the association reaches a plateau at a
> s.size of 10 and a bedekking of 0.45. So these are the values I use in
> 'psi' argument in the 'segmented' function
>
>> psi.mod<- list(0.45, 10)
>> names(psi.mod)<- c("bedekking", "s.size")
>
> Then I attempt to run the 'segmented' function:
>
>> seg.gm<- segmented(obj = gmodel, seg.Z= ~bedekking + s.size, psi =
psi.mod)
>
> When I run this, after half a minute I get the following error:
>
> Error in ifelse(is.list(o0), o0$dev.no.gap, 10^12) :
>    replacement has length zero
>
> Does anybody know what might be causing this error, and could somebody
> point out where I might go wrong?
>
> Thanks in advance,
>
> Peter
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
-- 
===================================Vito M.R. Muggeo
Dip.to Sc Statist e Matem `Vianelli'
Universit? di Palermo
viale delle Scienze, edificio 13
90128 Palermo - ITALY
tel: 091 23895240
fax: 091 485726
http://dssm.unipa.it/vmuggeo