Thank you, it works! And the estimates (as well as the standard errors) seem to be more reasonable now, compared to the "normal" Poisson model. Thank you. However, I tried to find a manual  (although I did manage to find the paper published in the Journal of Statistical Software. For example, how does ?zerotrunc return?

 
Dr. Iasonas Lamprianou
Department of Social and Political Sciences
University of Cyprus


>________________________________
>From: Achim Zeileis <Achim.Zeileis@uibk.ac.at>
>To: Iasonas Lamprianou <lamprianou@yahoo.com>
>Cc: Mitchell Maltenfort <mmalten@gmail.com>; "r-help@r-project.org" <r-help@r-project.org>
>Sent: Monday, 1 August 2011, 10:10
>Subject: Re: [R] zero truncated poisson regression
>
>On Sun, 31 Jul 2011, Iasonas Lamprianou wrote:
>
>> Thanks
>> Pscl seems to be a sensible option.
>>
>>
>> I have the counts variable with the name "N". This variable can only 
>> take values bigger than zero!
>>
>> I have two explanatory variables with the names "type" and "diam"
>>
>> but when I run
>>
>> hpm <- hurdle(n ~ type+diam, data = an, dist = "poisson")
>>
>> I get the message "invalid dependent variable, minimum count is not 
>> zero". Well, I know that N>0, that is why want to run a zero-truncated 
>> model. But I must be missing something...and the manual does not seem to 
>> help a lot...
>>
>> Can anyone help please?
>
>As previously pointed out by others on this list: hurdle() is not what you 
>are looking for (although it is related to what you want to do). The 
>hurdle() model is a two-part model consisting of a zero-truncated count 
>part and a binary part for modeling N=0 vs N>0. See also 
>vignette("countreg", package = "pscl") for details.
>
>As you don't need the binary hurdle part, you cannot use hurdle() 
>directly.
>
>This is why the package "countreg" on R-Forge provides the function 
>zerotrunc() which essentially does the same thing as the count part in 
>hurdle().
>
>install.packages("countreg", repos = "http://R-Forge.R-project.org")
>library("countreg")
>m <- zerotrunc(n ~ type + diam, data = an, dist = "poisson")
>summary(m)
>
>> ?
>> Dr. Iasonas Lamprianou
>> Department of Social and Political Sciences
>> University of Cyprus
>>
>>
>>> ________________________________
>>> From: Mitchell Maltenfort <mmalten@gmail.com>
>>> To: Iasonas Lamprianou <lamprianou@yahoo.com>; "r-help@r-project.org" <r-help@r-project.org>
>>> Sent: Sunday, 31 July 2011, 20:45
>>> Subject: Re: [R] zero truncated poisson regression
>>>
>>> Pscl package.
>>>
>>> On 7/31/11, Iasonas Lamprianou <lamprianou@yahoo.com> wrote:
>>>> Dear friends,
>>>>
>>>> does anyone know how I can run a zero truncated poisson regression using R
>>>> (or even SPSS)?
>>>>
>>>> Dr. Iasonas Lamprianou
>>>> Department of Social and Political Sciences
>>>> University of Cyprus
>>>>
>>>> ??? [[alternative HTML version deleted]]
>>>>
>>>>
>>>
>>> --
>>> Sent from my mobile device
>>>
>>> Due to the recession, requests for instant gratification will be
>>> deferred until arrears in scheduled gratification have been satisfied.
>>>
>>>
>>>
>>     [[alternative HTML version deleted]]
>>
>>
>
>
>
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