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

[Daniel Ezra Johnson]

Shrinkage is not the main issue, as I see it here. When the predictor  
of interest is Sex you should include by-subject random effect(s),  
when it's Frequency you should include by-item. Probably you should  
include both in both cases. You can't do accurate hypothesis testing  
on Sex and Frequency if you ignore the variation among Subjects and  
Items.

On Dec 8, 2009, at 2:15 AM, Antoine Tremblay <trea26 at gmail.com> wrote:

> 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
>
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