Thanks Sven , your explanations were helpful. As regards the invcdf function, according to Help, the invcdf function can be used for both the Binomial and the Poisson distributions. But it indeed seems not to work for either, I tried it for various parameter values. So perhaps the Help should be corrected? Alecos Papadopoulos Athens University of Economics and Business, Greece Department of Economics cell:+30-6945-378680 fax: +30-210-8259763 skype:alecos.papadopoulos On 16/10/2014 19:00, gretl-users-request(a)lists.wfu.edu wrote: