Dear List,

I tried to fit a mixed effect model.

data are like this:

group    id time   t
control  1    0      4.3
control  2    0      3.7
control  3    0      4.0
control  4    0      3.6
control  5    0      4.1
control  6    0      3.8
......
control  4    5      5.1
control  5    5      4.5
control  6    5      3.3

it has repeated measurement("t") for each "id" at different "time".

So I constructed a mixed effect model using package nlme:

m1<-lme(t~time*group,random=~time|id,data=p_long)

it worked fine. However when I used

m2<-lme(t~time*group,random=~time|id,data=p_long,method="ML")

it gave me error:
"Error in lme.formula(t ~ time * group, random = ~time | id, data = p_long,
:
  nlminb problem, convergence error code = 1
  message = iteration limit reached without convergence (9)"

and if I changed it to

m3<-lme(t~time*group,random=~1|id,data=p_long,method="ML")

It also worked fine.

In summary,

m1 used "random=~time|id" and no "ML" (so it was "REML"), it worked fine.
m2 used "random=~time|id" and "ML", it gave errors
m3 used "random=~1|id", and "ML", it worked fine.

Could someone explain these to me?

Thanks you.

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