Thanks dear Steve, I try by this: data have; input id sex$ age weight; cards; 1 F 10 20 1 F 22 60 2 M 9 15 2 M 18 54 ; proc nlmixed data=have; by id; parms a=60 b=5 k=0.5 s2e=1 s2u=10; ex=exp(-k*age); num=(a*(1-(b*ex)))+u; model weight~normal(num,s2e); random u ~ normal(0,s2u) subject=id; predict num out=output_fixed; run; Is it true (please assume weight has a normal distribution)? However, when i run this program i can't found any parameters in the output file (output_fixed). I need a program that estimate all parameters (A, B & K) for each ID in each age. Please check my program and let me i have your idea. Best regards Zana
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