I am a dentist (so forgive me if my question looks stupid) and came across a problem when I did simulations to compare a few single level and two level regressions.
The simulations were interrupted and an error message came out like 'Error in MEestimate(lmeSt, grps) : Singularity in backsolve at level 0, block 1'.
My collegue suggested that this might be due to my codes was not efficient and ran out of memory. If this is the reason, could you please help me improve my codes writing.
However, as I slightly changed the parameters, it ran well. So I suspect memory is problem.
I use R 1.8.0 and Windows XP professional. My computer has a Pentium 4 2.4 with 512 MB memory.
Thanks in advance.
best regards,
Yu-Kang Tu
Clinical Research Fellow Leeds Dental Institute University of Leeds
## change scores simulation
close.screen(all=TRUE)
split.screen(c(3,3))
nitns<-10000
nsims<-100
r<-0.1
param1<-c(1:nitns)
param2<-c(1:nitns)
param3<-c(1:nitns)
param4<-c(1:nitns)
param5<-c(1:nitns)
param6<-c(1:nitns)
param7<-c(1:nitns)
param8<-c(1:nitns)
param9<-c(1:nitns)
param10<-c(1:nitns)
param11<-c(1:nitns)
param12<-c(1:nitns)
param13<-c(1:nitns)
param14<-c(1:nitns)
for(itn in 1:nitns){
g<-rbinom(nsims,1,0.5)
b<-rnorm(nsims,0,1)*10
rn<-rnorm(nsims,0,1)*10
a<-b*r+rn*(1-r^2)^0.5
a<-round(a)+50
a<-a-g*5
b<-round(b)+50
abs.2<-function(x) ifelse(x<1,1,x)
b<-abs.2(b)
c<-b-a
p<-c/b
lm1<-lm(a~g)
lm2<-lm(c~g)
lm3<-lm(p~g)
lm4<-lm(a~b+g)
gr<-c(g,g)
occasion<-rep(0:1,c(nsims,nsims))
occ<-occasion-0.5
ppd<-c(b,a)
h<-rep(0,nsims)
mb<-mean(b)
bppd<-b-mb
bappd<-c(h,bppd)
occgr<-occ*gr
subject<-c(1:nsims)
sub<-c(subject,subject)
library(nlme)
lm5<-lme(ppd~occ+gr+occgr,random=~1|sub)
lm5f<-fixed.effects(lm5)
lm5c<-as.matrix(lm5f)
lm5a<-anova(lm5)
lm6<-lme(ppd~occ+gr+occgr,random=~1|sub,method="ML")
lm6f<-fixed.effects(lm6)
lm6c<-as.matrix(lm6f)
lm6a<-anova(lm6)
lm7<-lme(ppd~occ+gr+occgr+bappd,random=~1|sub,method="ML")
lm7f<-fixed.effects(lm7)
lm7c<-as.matrix(lm7f)
lm7a<-anova(lm7)
param1[itn]<-coef(summary(lm1))[2,1]
param2[itn]<-coef(summary(lm2))[2,1]
param3[itn]<-coef(summary(lm3))[2,1]
param4[itn]<-coef(summary(lm4))[3,1]
param5[itn]<-lm5c[4,1]
param6[itn]<-lm6c[4,1]
param7[itn]<-lm7c[4,1]
param8[itn]<-coef(summary(lm1))[2,4]
param9[itn]<-coef(summary(lm2))[2,4]
param10[itn]<-coef(summary(lm3))[2,4]
param11[itn]<-coef(summary(lm4))[3,4]
param12[itn]<-lm5a[4,4]
param13[itn]<-lm6a[4,4]
param14[itn]<-lm7a[4,4]
}
#the error message came out here.
#But if I change some of the variables to:
b<-rnorm(nsims,0,1)*2
rn<-rnorm(nsims,0,1)*2
a<-b*r+rn*(1-r^2)^0.5
a<-round(a)+7
a<-a-g*2
b<-round(b)+9
abs.1<-function(x) ifelse(x<5,5,x)
b<-abs.1(b)
abs.2<-function(x) ifelse(x<1,1,x)
a<-abs.2(a)There is no poblem to complete the simulations.
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