[R-sig-ME] The effects of adding by-subject or by-item random intercepts

[Antoine Tremblay]

Dear all,

This question is about the effects of adding by-subject or by-item
random intercepts to a model.

If we are contrasting a single condition between two subject groups,
say ReactionTime ~ Sex,
is it warranted (or necessary or ill-advised) to include by-subjects
random intercepts,
since this could (if I'm understanding it correctly) adjust the mean
reaction time for each subject (and thus for
each condition) towards the grand mean, thus reducing or
eliminating the difference in the condition between subjects? And
similarly if we are contrasting a
single condition between two sets of items, say ReactionTime ~ Frequency?

I believe that the addition of the random effect may reduce the effect
of the fixed effect, but should
not remove it entirely. Is this right?

The question would then become: Why would the addition of say by-item
random intercepts to a model
take away an effect that was present in a model without by-item random
intercepts?

Thank you again, your help is well appreciated.

-- 
Antoine Tremblay
Department of Neuroscience
Georgetown University
Washington DC