Dear All,
I am a researcher doing research in plant growth and I have a 
statistical problem that seems to not be able to handle. Recently, I 
conducted an experiment about plant growing in three different 
nutrient-level sediments. I harvested these every three week (three 
harvests in all). Some growth traits of these plants were recorded (e.g. 
total biomass, leaf biomass and stem biomass). In addition, I found that 
the total plant biomass may also influence these traits. So, I am going 
to use total biomass as covariate to do the statistical analysis. 
Meanwhile, this experimental design is repeated design, I guess I have 
to do the repeated-measurement  (repeated measure ANCOVA with varying 
covariate) to analyze the growth traits between different 
nutrient-levels. I've tried gls and lme function with a correlation 
structure applied to time series in nlme library with great help by Dr. 
Yuanye Zhang. However, I still can't figure out the "between-subject" 
and "with-subject" parts.

Do you know if R can do repeated measures ANCOVA with varying covariate? 
if yes how?

Thanks for any help. Much appreciated.

Dong Xie


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