[Rasch] Mean diferences and measurement error

Hoi Suen HoiSuen at psu.edu
Tue Jun 5 09:02:10 EST 2007


The "sampling error" referred to in Andres' original post was the error 
of sampling subjects. Your situation has to do with the error of 
sampling tasks. In fact, there are other additional "sampling errors"; 
such as the error of sampling raters, of occasions, of ... From the 
perspective of G-theory, measurement error is the error of sampling 
various facets/dimensions -- other than the dimension of the intended 
object of measurement (i.e., people).  To adjust for 
measurement/sampling error in your correlation of writing samples and 
other measures, I would suggest estimating the appropriate G-coefficient 
for the writing samples and for the other measures and then apply a 
standard correction for attenuation procedure.

(If these points about classical and G-theory is inappropriate for a 
Rasch listserv, I apologize.)


Donald Bacon wrote:
> I have a related question -- what about correcting for measurement 
> error when there is also sampling error?
> I've asked experts to count the number of errors in a writing sample.  
> I can compute the inter-rater correlation, and thus can estimate the 
> reliability of the measures (I'm working on doing this in Facets 
> also).  However, each sample of writing is just one sample that an 
> individual could produce.  Therefore, there is also sampling error, 
> which can be estimated by examining the distribution of errors within 
> the writing sample.  Small writing samples have higher sampling error 
> than large writing samples, even though the reliability may be the same.
> Now when I examine the correlation between these ratings of writing 
> samples and some other measure (I have an objectively-scored, 
> Rasch-modeled measure), and I want to correct for measurement error to 
> estimate the latent correlation, shouldn't I adjust for measurement 
> error and sampling error?  (Does anyone have a good cite on this?)
> Don Bacon
> Associate Professor of Marketing
> University of Denver
> ------------------------------------------------------------------------
> *From:* rasch-bounces at acer.edu.au [mailto:rasch-bounces at acer.edu.au] 
> *On Behalf Of *Hoi Suen
> *Sent:* Monday, June 04, 2007 1:35 PM
> *To:* Andrés Burga León
> *Cc:* rasch at acer.edu.au
> *Subject:* Re: [Rasch] Mean diferences and measurement error
> Andrés,
> I don't think it is necessary to assume perfect reliability in tests 
> of significance such as the simple t-test. Using concepts in the true 
> score model of measurement, the standard error of the mean in the 
> denominator of the simple t-test is the standard error of the mean _of 
> OBSERVED scores_. Since observed score is the linear combination of 
> true and measurement error scores, the standard error of the mean is 
> in fact already sqrt(sampling error^2 + measurement error^2). The 
> first term is the _sampling_ error of _true_ scores and the second 
> term is random measurement error. It is because of these two terms 
> that power in a t-test is reduced by heterogeneity (i.e., first term) 
> and unreliability (i.e., second term).
> This is also true within the conceptual framework of the 
> generalizability theory. In that case, the standard error of the mean 
> in the t-test is conceptually equivalent to the square root of the 
> expected mean error variance in G-theory.
> Hoi
> Andrés Burga León wrote:
>> In every statistical test, you found that if you wan't to assess mean 
>> differences you could use a simple t test: (mean1 -- mean 2) / 
>> standard error. But this formula only considers the sampling error. 
>> It assumes perfect reliability.
>> What about the measurement error? Why didn't consider it in the 
>> assessment of mean differences. I'm not expert in this subject, but 
>> could it be possible to made a linear composite of sampling error and 
>> measurement error? I mean something like sqrt(sampling error^2 + 
>> measurement error^2) and so assess better mean groups differences?
> -- 
> ------------------------------------------------------------------------
> Hoi K. Suen, Ed. D.
> Distinguished Professor
> Educational Psychology
> Penn State
> Website: suen.ed.psu.edu <http://suen.ed.psu.edu>

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