[R] Question about data used to fit the mixed model

[Nantachai Kantanantha]

Hi everyone,

I would like to ask a question regarding to the data used to fit the mixed 
model.

I wonder that, for the response variable data used to fit the mixed model 
(either via "spm" or "lme"), we must have several observations per subject 
(i.e. Yij,  i = 1,..,M,  j = 1,.., ni) or it can be just one observation per 
subject (i.e. Yi,  i = 1,...,M). Since we have to specify the groups for 
random effect components, if we have only one observation per subject, then 
each group will have only one observation.

Thank you vert much for your help.
Sincerely yours,

Nantachai