[R] Help in error of mixed models

[Bernardo Rangel Tura]

Hi R-masters

I read the article: Bivariate analysis of sensitivity and specificity
produces informative summary measures in diagnostic reviews.

In this paper i proposed a bivariate mixed model and use SAS proc mixed
to adjust the estimates.


I thinks use R to make the same and try with this code:

base<-read.csv("base.csv")
adj<-.5
attach(base)

sens<-(VP+adj)/(VP+FN+2*adj)
log.S<-log(sens/(1-sens))
var.log.S<-1/(sens*(1-sens)*(VP+FN))
dis<-rep(1,length(log.S))
non.dis<-rep(0,length(log.S))
data.S<-data.frame(id,Modality,log.S,var.log.S,dis,non.dis)
names(data.S)<-c("id","Modality","logit","var.logit","dis","non.dis")

esp<-(VN+adj)/(VN+FP+2*adj)
log.E<-log((1-esp)/esp)
var.log.E<-1/(esp*(1-esp)*(VN+FP))
dis<-rep(0,length(log.E))
non.dis<-rep(1,length(log.E))
data.E<-data.frame(id,Modality,log.E,var.log.E,dis,non.dis)
names(data.E)<-c("id","Modality","logit","var.logit","dis","non.dis")

data.bi<-rbind(data.S,data.E)
require(nlme) 
lme(logit~dis*Modality+non.dis*Modality, random=~dis|id+non.dis|
id,data=data.bi)

but i recive a erro msg :

Error in MEEM(object, conLin, control$niterEM) : 
  Singularity in backsolve at level 0, block 1


How in solve this problem? Whats is wrong?

Thanks in advance
-- 
Bernardo Rangel Tura, M.D,Ph.D
National Institute of Cardiology
Brazil