[R-sig-ME] How to select (and nest) covariates in a crossed design model?

[Mari Laine]

Hi all,

I have difficulties in building a proper model for my experimental data. The study was done in two areas, A and B. A received a test treatment and B acted as a control, and then the experiment was repeated with inversed treatments, making it a crossed design. Response was measured in three sessions per treatment per area: before, during and after the treatment. Each session included 3 consecutive observations (called rounds).

I'm interested in the impact of treatment (test vs. control), and whether it is related to sessions or not. I've built a generalized linear mixed model, and placed "treatment", "session" and their interaction as fixed factors. My problem with the model is that, in addition to "experiment", I'm not quite sure what to include in random (or repeated?) factors and whether to nest these factors or not. I've browsed the web for examples that would include using a 2x3 (or 2x2) crossed design with multiple observations within a cell, but I've found none.


Here is a data table resembling the one I'm using in my analysis:

Area A
experim	treatm	before			during			after
1	c	1.80	0.44	0.71	2.08	1.10	1.48	2.45	1.64	3.88
2	e	6.11	4.66	5.40	6.98	4.57	5.39	7.83	6.31	7.25

Area B
experim	treatm	before			during			after
1	e	1.96	2.08	2.20	0.92	1.34	1.42	0.59	1.02	1.76
2	c	1.45	2.41	4.62	2.55	5.31	3.75	5.03	8.24	1.28


And the same data in an importable form:
area	experim	treatment	session	sessnro	round	response
A	1	c	before	1	1	1.80
A	1	c	before	1	2	0.44
A	1	c	before	1	3	0.71
A	1	c	during	2	4	2.08
A	1	c	during	2	5	1.10
A	1	c	during	2	6	1.48
A	1	c	after	3	7	2.45
A	1	c	after	3	8	1.64
A	1	c	after	3	9	3.88
A	2	e	before	4	10	6.11
A	2	e	before	4	11	4.66
A	2	e	before	4	12	5.40
A	2	e	during	5	13	6.98
A	2	e	during	5	14	4.57
A	2	e	during	5	15	5.39
A	2	e	after	6	16	7.83
A	2	e	after	6	17	6.31
A	2	e	after	6	18	7.25
B	1	e	before	1	1	1.96
B	1	e	before	1	2	2.08
B	1	e	before	1	3	2.20
B	1	e	during	2	4	0.92
B	1	e	during	2	5	1.34
B	1	e	during	2	6	1.42
B	1	e	after	3	7	0.59
B	1	e	after	3	8	1.02
B	1	e	after	3	9	1.76
B	2	c	before	4	10	1.45
B	2	c	before	4	11	2.41
B	2	c	before	4	12	4.62
B	2	c	during	5	13	2.55
B	2	c	during	5	14	5.31
B	2	c	during	5	15	3.75
B	2	c	after	6	16	5.03
B	2	c	after	6	17	8.24
B	2	c	after	6	18	1.28


Thanks,
 Mari Laine