Hi, I was reviewing a manuscript where a linear mixed model was used. The data is simple: a response variable "y" was measured for each subject over 3 time points (visit 1, 2 and 3) that were about a week apart between 2 visits. The study is a non-drug study and one of the objectives was to evaluate the repeatability of response variable "y". 


The author wanted to estimate within-subject variance for that purpose. This is what he wrote "within-subject variance was generated from SAS 'Prog Mixed' procedure with study visit as fixed effect and subject as random effect". I know that the study visit was a factor variable, not a numeric variable. Because each subject has 3 repeated measurements from 3 visits, how can a model including subject as random effect still use visit as fixed factor? If I would do it in R, I would just use a simple model to get within-subject variance:

obj<-lmer(y~1+(1|subject),data=data)

What does a model "obj<-lmer(y~visit+(1|subject),data=data)" mean?

appreciate any thoughts!

John

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