Parameter values in proc Nlmixed

[Abbas_Arkawazi]

Hello everyone...please help me in how to chose the initial values for the parameters in the proc Nlmixed for the following model with attached data.

proc import datafile='C:\Users\Abbas Arkawazi\Desktop\test.xlsx'
DBMS =xlsx Out=test;
run;
proc nlmixed data=test;
parms b0=70 b1=1 b2=8 b3=1 a=2 b=1 s=220;
bounds a b > 0;
m =b0+(b1*age)+(b2*creatin)+(b3*gender1);
y=urea;
if status_u=1 then f=((a*b)*((1-exp(-exp((y-m)/s)))**(a-1))*((1+((exp(exp((y-m)/s))-1)**(a)))**(-(b+1)))) /(s*exp((-a*exp((y-m)/s))-((y-m)/s))) ;
else f= (1+(((1-exp(-exp((y-m)/s)))/(exp(-exp((y-m)/s))))**a))**-b ;
ll=log(f);
model y ~ general(ll);
run;