Mixture model for Disease Progression

Hello, I am trying some simple linear disease progression analysis and the data suggests that there are 2 populations in the dataset (Low Baseline, Low Slope vs. High Baseline, High Slope). It appears that that population consists of progressers and non-progressers (The Pharmacometrics textbook describes some code with 3 populations i.e. positive slope, zero slope, and negative slope but I want to keep my model simple). This is the only piece of the code that seems to work for my dataset. If I try to estimate THETA(3) in my code it causes the model to either becomes unstable or I get a very small negative value for THETA(3) with very poor precision. I would really appreciate feedback from NMusers on the parameterization of the slope parameter in this model (thetas and etas for the 2 populations). Many thanks in advance...MNS $PK CALLFL =1 EST=MIXEST IF (MIXNUM.EQ.2) THEN BASE=THETA(2)*EXP(ETA(2)) SLOP=THETA(4)+ETA(4) ELSE BASE=THETA(1)*EXP(ETA(1)) ; PATHOLOGIC BASELINE SLOP=THETA(3)*(1+ETA(3)) ; PATHOLOGIC PROGRESSION PARAMETER; ETA PARAMETERIZATION BASED ON TUTURIAL FROM PAGE ENDIF $THETA (0,15 ) ; BASELINE PATHOLOGIC (0,7) ; BASELINE NON-PATHOLOGIC (0,0.1) ; SLOPE PATHOLOGIC (0 FIX ) ; SLOPE NON-PATHOLOGIC $OMEGA BLOCK(1) 0.15 $OMEGA BLOCK(1) SAME $OMEGA BLOCK(1) 0.35 $OMEGA BLOCK(1) 0.03