```Date: Tue, 5 Mar 2002 13:41:02 -0800 Reply-To: Dale McLerran Sender: "SAS(r) Discussion" From: Dale McLerran Subject: Re: Genmod and Positive Poisson In-Reply-To: Content-Type: text/plain; charset=us-ascii Silvano, What is the Positive Poisson distribution? How does it arise? If a Positive Poisson distribution is a Poisson distribution with zero values censored so that the probability function could be written as p(y) = exp(-Xbeta)*(Xbeta**y)/(y!) / (1 - exp(-Xbeta)*(Xbeta**0)*(0!)) = exp(-Xbeta)*(Xbeta**y)/(y!) / (1 - exp(-Xbeta)) then you could use the nlmixed procedure in version 8.x of SAS to fit the model without having to specify deviance and variance functions. All that you need to specify for nlmixed to work is the likelihood function. The following syntax should fit the model: proc nlmixed data=mydata; Xbeta = beta0 + x1*beta1 + x2*beta2 + ... + x*beta; p_y = exp(-Xbeta)*(Xbeta**y)/(fact(y)) / (1 - exp(-Xbeta)); loglike = log(p_y); model y ~ general(loglike); run; If my assumption about the probability function of the Positive Poisson distribution is incorrect, then simply modify the probability function to provide the correct form. And please respond to the first couple of questions above. I really would like to know just what this distribution is and how it occurs. Dale --- Silvano Cesar da Costa wrote: > Hi, > > I'm trying fit Positive Poisson using Genmod. > The statements are: > > Proc Genmod; > class ini cult; > m = _mean_; > a = m/(1-exp(-m)); > y = _resp_; > d = 2 * (y * log(y/a) + log( (exp(a)-1)/(exp(y)-1))); > variance var = a * (1 + m -a); > deviance dev = d ; > model nbrotos = ini|cult / link=log type1 type3; > run; > > but, I'm not sure if was correct. > > Someone can help me. > > Thanks, > > Silvano. ===== --------------------------------------- Dale McLerran Fred Hutchinson Cancer Research Center mailto: dmclerra@fhcrc.org Ph: (206) 667-2926 Fax: (206) 667-5977 --------------------------------------- __________________________________________________ Do You Yahoo!? Try FREE Yahoo! Mail - the world's greatest free email! http://mail.yahoo.com/ ```

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