[R] Crossed random effects
jonathan.williams at pharmacology.oxford.ac.uk
Thu Apr 25 11:50:57 CEST 2002
I would be very grateful for help with devising the correct formula
for an analysis with crossed random effects.
The data consist of measurements of the width of a brain structure
from 145 subjects, on both the right and left. So, Side is nested
within Subject in a balanced design.
3 raters measured the widths. One rater measured all 145 widths twice
- so there were 2 replications. One rater measured all the widths,
and then re-measured a random 30% of them. One measured only a random
70% of the widths once. So, replication is nested within rater, and
these random factors are highly unbalanced.
I want to analyse the relationship of the widths to subject variables
such as age and gender. But, I do not know how to write the correct
formula for the random effects to specify how the Subject/Side structure
is crossed with the Rater/Replication structure. I gather from reading
Pinheiro and Bates (p163) that "The crossed random effects structure is
represented in lme by a combination of pdBlocked and pdIdent objects".
But, I cannot see how to extend the example in the book to fit my data.
I would be very grateful for help with this.
OPTIMA Clinical Fellow and Honorary Consultant Psychiatrist
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