[R-sig-ME] Rasch with lme4
ccleland at optonline.net
Tue Jun 9 18:25:40 CEST 2009
On 6/9/2009 6:59 AM, Andy Fugard wrote:
> Dear all,
> What happens in practice when you compare the two approaches of item as
> a fixed versus as a random effect?
> M1 = lmer(Reaction ~ Days + (1|Subject), sleepstudy)
> M2 = lm(Reaction ~ Days + factor(Subject), sleepstudy)
> The slope estimates for Days for are practically identical, the mean
> intercepts differ:
> For M1:
> Fixed effects:
> Estimate Std. Error t value
> (Intercept) 251.4051 9.7459 25.80
> Days 10.4673 0.8042 13.02
> For M2:
> Estimate Std. Error t value Pr(>|t|)
> (Intercept) 295.0310 10.4471 28.240 < 2e-16 ***
> Days 10.4673 0.8042 13.015 < 2e-16 ***
> I didn't look at the estimators for Subject, e.g., for M2 the predictors:
> factor(Subject)309 -126.9008 13.8597 -9.156 2.35e-16 ***
> factor(Subject)310 -111.1326 13.8597 -8.018 2.07e-13 ***
> factor(Subject)330 -38.9124 13.8597 -2.808 0.005609 **
> But it could be done...
> Is there a paper on these sorts of comparisons? How does the mixed
> effects approach differ from a standard regression model with a heap of
> categorical predictors for representing, e.g., deviations from the mean
Allison, P.D. (2005). Fixed Effects Regression Methods for Longitudinal
Data Using SAS. Cary, NC: SAS Institute.
covers some of this territory. As someone else pointed out, a
limitation is that including "Subject" as a factor precludes inclusion
of specific subject explanatory variables (e.g., gender).
> Presumably this could be done too for estimates for items, e.g., for
> binary logistic models and beyond.
> Ken Beath wrote:
>> On 09/06/2009, at 8:58 AM, Stuart Luppescu wrote:
>>> On 火, 2009-06-09 at 08:04 +1000, Ken Beath wrote:
>>>> The model treats item as a random effect and should be a fixed effect.
>>> Hmm. In Doran, Bates, Bliese and Dowling (2007), the authors treat the
>>> item as random.
>> It can be argued that the items are a sample from a population of
>> items which is possibly reasonable for educational testing where there
>> might be a population of questions which can be asked. Even so,
>> assumptions about the distribution are optimistic and most items are
>> used because they test something obvious. Maybe others have a
>> different philosophy. A more pedantic argument is that this isn't the
>> model Rasch used.
>>>> Another question to ask is whether the Rasch model is appropriate. If
>>>> an IRT is more sensible it would cause some problems with the second
>>> Sorry, but I don't understand this at all.
>> By an IRT I mean the 2 parameter version where there is a discriminant
>> parameter which varies among items, in contrast to the Rasch where it
>> is constant. It probably gives problems with the other model as well
>> but the second model should have more problems.
>> I don't like the idea of assuming a Rasch model at all, its popularity
>> seems to derive from an era when fitting anything else was difficult.
>> Modern software offers proper solutions, unfortunately at a cost but
>> that shouldn't be a consideration.
>>> Stuart Luppescu -=- slu .at. ccsr.uchicago.edu
>>> University of Chicago -=- CCSR
>>> 才文と智奈美の父 -=- Kernel 2.6.28-gentoo-r5
>>> Drusilla: How do you feel about eternal life?
>>> Xander: We couldn't just start with coffee?
>> R-sig-mixed-models at r-project.org mailing list
Chuck Cleland, Ph.D.
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