```Date: Wed, 14 Nov 2007 11:07:16 -0800 Reply-To: Robin High Sender: "SAS(r) Discussion" From: Robin High Subject: Re: a question regarding genmod Comments: To: wei yi In-Reply-To: A<546754.70408.qm@web34304.mail.mud.yahoo.com> Content-Type: text/plain; charset="US-ASCII" Vivian, What you should consider is a key point about the Poisson distribution - that is, it's a probability function based on integer counts, so the sums of the probabilities must add to 1 for integers from 0 to infinity. There are (at least) 3 ways to compute the Poisson probs shown below .. 2 of them fail for 'non-integer' values and of course, the cumulative sum is greater than 1 for prb2, based on the GAMMA() function. * poisson with mean=3; data one; cm_prb1=0; cm_prb2=0; do y = 0 to 20 by .5; prb1 = exp(-3)*(3**y)/FACT(y); prb2 = exp(-3)*(3**y)/GAMMA(y+1); prb3 = pdf('poisson',y,3); cm_prb1+prb1; cm_prb2+prb2; output; end; proc print; var y prb1 cm_prb1 prb2 cm_prb2 prb3; run; Thus, the PDF the the POISSON is a function you can maximize for non-integer values, though with non-integer values it doesn't retain one of the necessary characteristics that define it to be a probability distribution. If you have non-integer data (and yes, I'm curious what they are) you perhaps would do reasonably well to utilize the normal (i.e., a continuous) approximation (or some other continuous choice with GENMOD), depending on the actual distribution of y. Robin High Univ. of Oregon -----Original Message----- From: SAS(r) Discussion [mailto:SAS-L@LISTSERV.UGA.EDU] On Behalf Of wei yi Sent: Wednesday, November 14, 2007 8:29 AM To: SAS-L@LISTSERV.UGA.EDU Subject: a question regarding genmod I have a question regarding running a GLM with distribution of poisson . proc genmod data=test; class x1 x2; model y=x1 x2 x3 x4 x5/link=log dist=poission; run; my question is whether it is OK when variable Y could be a decimal number but not an interger if I run in SAS. Thanks, ```

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