Hi,

I am trying to convert the following NLMIXED code to NLME, but am
running into problems concerning 'Singularity in backsolve'. As I am new
to R/S-Plus, I thought I may be missing something in the NLME code.

NLMIXED
***********
proc nlmixed data=kidney.kidney;
parms delta=0.03 gamma=1.1 b1=-0.003 b2=-1.2 b3=0.09 b4=0.35 b5=-1.43
varu=0.5;
eta=b1*age+b2*sex+b3*gn+b4*an+b5*pkn+u;
hazard=eta+log(delta)+log(gamma)+(gamma-1)*log(rtime);
survivor=(-exp(eta))*delta*(rtime**gamma);
ll=(event*hazard)+survivor;
model rtime ~ general(ll);
random u~normal(0,varu) subject=patient out=frailty;
run;

NLME
********
kidney.nlme<-nlme(model=rtime~
(event*
((b1*age+b2*sex+b3*gn+b4*an+b5*pkn+u)+log(delta)+log(gamma)+(gamma-1)*lo
g(rtime))
+((-exp(b1*age+b2*sex+b3*gn+b4*an+b5*pkn+u))*delta*(rtime**gamma))
),
fixed=list(delta~1,gamma~1,b1~1,b2~1,b3~1,b4~1,b5~1),
random=u~1|patient,
start=c(delta=0.03,gamma=1.1,b1=-0.003,b2=-1.2,b3=0.09,b4=0.35,b5=-1.43)
,
data=(kidney),
method="ML",
na.action=na.include
)

Error in NLME
*************
> traceback()
11: eval(action, sys.parent())
10: doErrorAction("Problem in .C(\"fit_nlme\",: Singularity in
backsolve", 1000)
9: .C("fit_nlme",
8: nlme(model = rtime ~ (event * ((b1 * age + b2 * sex + b3 * gn + b4 *
an + b5 *
7: NULL
6: nlme.formula(model = rtime ~ (event * ((b1 * age + b2 * sex + b3 * gn
+ b4 *
5: eval(i, local)
4: source(auto.print = auto.print, exprs = substitute(exprs.literal))
3: script.run(exprs.literal = {
2: eval(expression(script.run(exprs.literal = {
1: 
Message: Problem in .C("fit_nlme",: Singularity in backsolve 

I am actually trying to fit a parametric model to the kidney catheter
data and compare NLMIXED with NLME. I am aware that COXPH and SURVREG
are also available with a frailty element added in, but wanted to fit
the likelihood model as above for a direct comparison.

Cheers,

Jindi

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