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Date:   Mon, 21 Mar 2005 20:32:04 -0800
Reply-To:   Dale McLerran <stringplayer_2@YAHOO.COM>
Sender:   "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
Comments:   DomainKeys? See
From:   Dale McLerran <stringplayer_2@YAHOO.COM>
Subject:   Re: ZIP models
In-Reply-To:   6667
Content-Type:   text/plain; charset=us-ascii

--- Jeffrey Stratford <stratja@AUBURN.EDU> wrote: > Hi. Two questions regarding ZIP models. > > 1. Some of my coefficients are negative for the covariates included > in > the zero-inflation part of the model. I understand a positive > coefficient - increasing the covariate means an increased probability > of > occurring in the zero class. But how do I translate a negative > coefficent? > > For example, I'm looking at the effect of urban cover on avian > density > and 12/14 species have the zero-inflation possitively associated with > urban cover. So I'm translating this to mean that the excess of > zeros > in my data are explained by urban cover and it turns out that birds > are > missing from highly urbanized sites. However, two species have a > negative coefficient for urban cover but what does that suggest? >

First of all, the zero-inflation parameters need to be interpreted in light of the Poisson mean expectation model. Assuming that the mean expectation model for the urban cover areas have equal or greater mean than the mean expectation for the non-urban areas, then the observation that the urban effect is negative means that urban areas have fewer areas in which there are non-Poisson zero values. Loosely, that would suggest that these species might flourish in urban areas. I believe that there are some bird species which do flourish in urban areas. Pigeons come to mind, but it seems to me that I have heard of other species too. Certainly there are mammals which are not adverse to human presence but in fact thrive in the presence of human activity. I don't know what species you are studying, but you might have to either reconsider your model (collinearity, left out variables, etc.) or your assumptions.

> 2. How does one evaluate model fit when SAS only gives a > -2loglikelihood? Is there a way to convert this to get a chi-square? >

I don't know of a likelihood ratio goodness-of-fit test for the ZIP model. You could compare the likelihood of the ZIP model with the likelihood of the Poisson model. The Poisson model is nested in the ZIP model. Thus, you can test whether the zero- inflation assumption results in an improved fit compared to a simple Poisson model.

> Thanks, > > Jeff > > **************************************** > Jeffrey A. Stratford, Ph.D. > 331 Funchess Hall > Department of Biological Sciences > Auburn University > Auburn, AL 36849 > 334-329-9198 > FAX 334-844-9234 > > **************************************** >

--------------------------------------- Dale McLerran Fred Hutchinson Cancer Research Center mailto: Ph: (206) 667-2926 Fax: (206) 667-5977 ---------------------------------------

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