Mixture model simulation

Hi, I've been puzzled by the behaviour of NONMEM when trying to simulate with a mixture model. The data file has 1000 subjects. I simulate 100 times (see control stream below). The overall number of subjects with MIXEST of 1 is 45%. There are no simulated values of 99. All values are either 1 or 2. I would expect about 50% of subjects to have MIXEST=1 in each replication. There are many replications of 1000 subjects which do not have MIXEST=2. This occurs with NONMEM 7.2.0 and both Intel and gfortran compilers. What is the expected behaviour of MIXEST when ICALL.EQ.4? Nick -------mixtest.ctl--------- $PROB mixbug $DATA mixtest.csv $INPUT ID DV $THETA 0.5 $OMEGA 0 FIX $SIGMA 1 FIX $PRED IF (NEWIND.LE.1) MIXE=99 IF (ICALL.EQ.4) MIXE=MIXEST REP=IREP Y=THETA(1)+ETA(1)+EPS(1) $MIX NSPOP=2 P(1)=THETA(1) P(2)=1-THETA(1) $TABLE REP ID MIXE NOAPPEND ONEHEADER NOPRINT FILE=mixbug.fit $SIM (1) ONLYSIM NSUB=100 -----mixtest.csv---- #ID, DV 1,. 2,. ... 1000,. -- Nick Holford, Professor Clinical Pharmacology Dept Pharmacology & Clinical Pharmacology, Bldg 503 Room 302A University of Auckland,85 Park Rd,Private Bag 92019,Auckland,New Zealand tel:+64(9)923-6730 fax:+64(9)373-7090 mobile:+64(21)46 23 53 email: [email protected] http://www.fmhs.auckland.ac.nz/sms/pharmacology/holford