[R] How to deal with this random variable?

Bert Gunter gunter.berton at gene.com
Mon Jul 27 20:04:54 CEST 2009


This sounds way too complicated for this forum, which is designed to provide
help to users on the  use of the R language, not remote statistical
consulting. While you may receive replies, I would argue that you would do
better to find a local statistical expert with whom to work -- not least
because they should probably have a deep understanding of how your
experiment was conducted, data gathered, measurements made, etc. to be able
to give you worthwhile advice.

Long distance consulting based on incomplete understanding is very risky.
Caveat emptor!

Bert Gunter
Genentech Nonclinical Biostatistics


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of Manuel Ramon
Sent: Monday, July 27, 2009 9:54 AM
To: r-help at r-project.org
Subject: [R] How to deal with this random variable?


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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