[R] How to deal with this random variable?

[Manuel Ramon]

Hello to everybody,
I have a data frame with 100 measures of quality for 3 variables: A, B and
C. These quality variables are measured in diferent times along the
productive process. My data comes from 5 experiments (5 replicates with 20
measures for replicate). I also have a final measure (Z) but just one
measure for each unit, that is, for the 20 units that are measured on each
replica. 

My objetive is to study the relationships between the 3 quality parameters
with the last measure, that is:
 
  lm(Z ~ A+B+C, data=mydata)

I have found significant differences between replicas for each qualite
parameters (A, B and C) and I would like to include the replica effect as a
random effect:

  lme(Z ~ A+B+C, data=mydata, random=~1|replica)

And here is my problem. I know that there are signifficant diferences
between replicas but since the final measure, Z, is the same for each
replica I do not know how to deal with. 

Can you help me? How could I take into account the variability due to the
replica when I want to study the effects of variables A, B and C on the
final result of a productive process?

Thank you in advance.

-----
Manuel Ramón Fernández
Group of Reproductive Biology (GBR)
University of Castilla-La Mancha (Spain)
mramon at jccm.es
-- 
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