[R-sig-ME] Design question about repeated measures as nested vs crossed structures

[Ista Zahn]

Sorry for the off-topic post, I've been struggling to understand
something and don't know where else to turn. I don't understand the
distinction between nested and cross classified, and I'd really
appreciate if someone can take a moment to set me straight. The
example below illustrates my confusion.

I often read/hear multivariate measures data described as nested, but
this doesn't make sense to me. Here is a typical explanation from
http://www.cmm.bris.ac.uk/lemma/mod/lesson/view.php?id=255:

"Sometimes we may wish to model more than one response. For example,
we may wish to consider jointly English and mathematics exam scores
for students because the two responses are likely to be related. We
can regard this as a multilevel structure with subjects (English and
maths) nested within students as shown in Figure 4.5. ..." (the figure
is here: http://www.cmm.bris.ac.uk/lemma/file.php/13/images-C4/image007.gif).

To my mind this sounds cross-classified, because each observation is a
particular combination of person and exam subject. It seems to make
just as much sense to describe these data as participant nested within
exam subject, as I've diagrammed here:
http://ista.scp.rochester.edu/snapshot1.png.

Please, if anyone can clear this up for me I'd really appreciate it.

-Ista

-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org