I used RooSimultaneous pdf to generate toy in this way: RooDataHist *asimovData0 = combPdf->generateBinned(RooArgSet(*obstmp, *m_cat), NumEvents(n_datatot),Extended(),Asimov()); here, the simultaneous pdf includes 3 category, and in each category, there are different observed variable, So obstmp set includes 3 variables. n_datatot = 7759, but when I printed out the information of the generated RooDataHist, I got: asimovData0->numEntries() = 3000000 asimovData0->sumEntries() = 7759 why the number of entries is so large. It causes the speed of next step of fitting this toy data is very slow. Is there any suggestion? Many thanks.