[R-lang] Re: Random slopes

[T. Florian Jaeger]

Hi Garry,

You may also find this informal description of the proposed model
comparisons helpful:
http://hlplab.wordpress.com/2009/05/14/random-effect-structure/

<http://hlplab.wordpress.com/2009/05/14/random-effect-structure/>florian

On Sun, Jan 2, 2011 at 5:15 PM, Levy, Roger <rlevy@ucsd.edu> wrote:

> OK, Garry, that helps a lot!  So, crucially, your model comparison is
> really a step in a larger effort to find which random-effects structure to
> use in drawing inferences about fixed effects.
>
> In this case, my suggestion is to start by skipping the model-comparison
> stuff and see if you can fit the full model
>
>  response ~ A*B + (A*B | subjects) + (A*B | items)
>
> If you are able to fit that (i.e. lme4 doesn't complain about singular
> convergence and gives you sensible-looking random-effects covariance
> matrices) and get significant fixed effects of A, B, and/or their
> interaction, then the result is maximally trustworthy.
>
> The trouble comes when you aren't able to fit that full model, which is
> *often*.  If this is the case, then stepwise model selection is a reasonable
> way to go, and Alex's advice (that is, always include the A*B fixed effect,
> even when testing whether to introduce just the random slope A) is sound.
>
> Best & good luck!
>
> Roger
>
>
>
> On Jan 2, 2011, at 4:05 PM, Zhenguang Cai wrote:
>
> > Hi Roger,
> >
> > Thanks. I am interested in average fixed effects.
> >
> > Garry
> >
> > 于 2011-1-2 20:28, Levy, Roger 写道:
> >>
> >> On Jan 1, 2011, at 4:56 PM, Zhenguang Cai wrote:
> >>
> >>> Dear Roger and Alex,
> >>>
> >>> Thanks for the replies. My goal of doing the model comparisons is to
> >>> determine whether to include random slopes (besides the random subject
> >>> and item intercepts) in my model.
> >>
> >> Hi Garry,
> >>
> >> Yes, this makes sense, but what is the ultimate goal of your modeling
> effort?  Are you ultimately trying to answer whether there are meaningful
> inter-participant differences in sensitivity to your factor A?  Are you
> trying to figure out whether the "average" (fixed) effect of A is non-zero?
>  Are you ultimately interested in whether there's an "average"
> (fixed-effect) interaction between A and B?  Or something else?
> >>
> >> Best
> >>
> >> Roger
> >>
> >>>
> >>> Thanks,
> >>>
> >>> Garryu
> >>>
> >>> 于 2011-1-1 22:15, Levy, Roger 写道:
> >>>>
> >>>> On Dec 30, 2010, at 5:59 PM, Alex Fine wrote:
> >>>>
> >>>>>
> >>>>>
> >>>>> Zhenguang Cai wrote:
> >>>>>> In determining whehter to include
> >>>>>> the by-subject slope for the main effect of A (in a 2*2 design),
> shall
> >>>>>> I just compare two simple models with A as the predictor only, as in
> >>>>>> the following:
> >>>>>>
> >>>>>> model1<- lmer (Y~A+(1|Subject)+(1|Item), data=dat, family=binomial)
> >>>>>> model2<- lmer (Y~A+(A+1|Subject)+(1|Item), data=dat,
> family=binomial)
> >>>>>>
> >>>>>> or shall I do it between two full models, as the following:
> >>>>>>
> >>>>>> model1<- lmer (Y~A*B+(1|Subject)+(1|Item), data=dat,
> family=binomial)
> >>>>>> model2<- lmer (Y~A*B+(A+1|Subject)+(1|Item), data=dat,
> family=binomial)
> >>>>> if the model you're interested in working with is Y ~ A*B, then you'd
> >>>>> want to do the latter comparison.  that way you're asking if a random
> >>>>> by-subject slope for A is justified for the model you're interested
> in
> >>>>> using to analyze your data.  if you're doing stepwise regression and
> >>>>> adding one predictor at a time, then it's more complicated.  but you
> >>>>> already said it's a 2x2 design, so it sounds like this isn't
> relevant.
> >>>>>
> >>>>> right, everyone?
> >>>>
> >>>> Garry:
> >>>>
> >>>> I think Alex is hitting the nail on the head in the sense that you
> haven't made completely clear what the goal of your statistical analysis is.
>  Usually it's easier to determine what model comparisons to make once the
> modeling goals are articulated fully.
> >>>>
> >>>> Best&   Happy New Year to all!
> >>>>
> >>>> Roger
> >>>>
> >>>> --
> >>>>
> >>>> Roger Levy                      Email: rlevy@ucsd.edu
> >>>> Assistant Professor             Phone: 858-534-7219
> >>>> Department of Linguistics       Fax:   858-534-4789
> >>>> UC San Diego                    Web:   http://idiom.ucsd.edu/~rlevy
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>
> >>> --
> >>> The University of Edinburgh is a charitable body, registered in
> >>> Scotland, with registration number SC005336.
> >>>
> >>
> >> --
> >>
> >> Roger Levy                      Email: rlevy@ucsd.edu
> >> Assistant Professor             Phone: 858-534-7219
> >> Department of Linguistics       Fax:   858-534-4789
> >> UC San Diego                    Web:   http://idiom.ucsd.edu/~rlevy
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >
> > --
> > The University of Edinburgh is a charitable body, registered in
> > Scotland, with registration number SC005336.
> >
>
> --
>
> Roger Levy                      Email: rlevy@ucsd.edu
> Assistant Professor             Phone: 858-534-7219
> Department of Linguistics       Fax:   858-534-4789
> UC San Diego                    Web:   http://idiom.ucsd.edu/~rlevy
>
>
>
>
>
>
>
>
>
>
>
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