🚀 go-pugleaf

In article <1992Oct15.184712.9572@vax.oxford.ac.uk>, jnewman@vax.oxford.ac.uk writes:
> 
> (1) Are there times in which blocks are clearly fixed effects and
> times in which they are clearly random effects? 

Yes. You stated the conditions.  It depends on how you want to generalize.
In my experience, they're almost always random.

> 
> Following up on this are two questions relating to the case in which
> blocks are considered random effects: 
> 
> (2) Agricultural experimental design texts tend to recommend
> purposefully confounding the interaction and the subjects within
> treatments term by having only a single subject per treatment by
> block combination. This circumvents the issue of whether blocks are
> fixed or random effects because only one possible error term exists.
> Why is it `correct' to assume no interaction term in this case but not
> when the design is not confounded?  Or, is it always our option to
> forget about the A*B interaction and use an error term with 85 df? 
> 

I'm not completely clear on what this says.  In agricultural randomized 
blocks experiments there is one observation per combination of treatment
and block, so the error term is the block*treatment interaction.

Do *not* forget about the interaction unless you've got good reason to do so.

> (3) If the A*B interaction is included in the model but not found to
> be significant, can we substitute the subjects within treatment term
> in forming the F ratio? 
> 

Do *not* pool unless you've got good reason to do so.  You could try the
analysis both ways.  If pooling makes a difference, be wary. In theory,
pooling in the absence of an interaction only increases your error df.  They've
got to be small to begin with for pooling to matter.

Also, be wary of any technique that applies a significant test conditionally
on the results of a prior test.

Sorry for the "sound bite" answers but, as you've alredy noted, entire
textbooks can be written about these issues.



> Thank you in advance for taking the time to answer these questions.
> 
> Jonathan Newman			Joy Bergelson
> Dept. of Zoology		Dept. of Biology
> University of Oxford, UK	Washington University, USA