--- a
+++ b/poisson.m
@@ -0,0 +1,26 @@
+# Copyright (C) 2005  Michael Creel michael.creel@uab.es
+# 
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ 
+# Example likelihood function (Poisson for count data) with score
+
+function [log_density, score] = poisson(theta, data, otherargs)
+	y = data(:,1);
+	x = data(:,2:columns(data));
+	lambda = exp(x*theta);
+	log_density = -lambda + y .* (x*theta) - lgamma(y+1);
+	score = dmult(y - lambda,x);
+	if (otherargs{1} == 1) score = "na"; endif # provide analytic score or not?
+endfunction