[Rasch] Fwd: Reliability, separation and strata

Fabio La Porta fabiolaporta at mail.com
Wed Mar 1 20:35:29 AEDT 2017

Hi Mike,
thanks for your explanation, I have understood now.

Just the last question: the Ben Wright's method can work also on a
distribution of persons approaching normality?

I am asking this because I like very much the possibility to define
graphically the ranges of the statistically different levels of performance
along the measurement continuum.

On the other hand, I have always find it difficult to draw strata on the
measurement continuum, as they have, almost always, decimal values, making
the whole process rather inaccurate.


2017-03-01 10:15 GMT+01:00 Mike Linacre <mike at winsteps.com>:

> Fabio:
> G and H are described at http://www.rasch.org/rmt/rmt94n.htm
> <https://api.thetopinbox.com/track/v2/link?id=00043403-6ad8-4f39-86a8-e0673884d97d&url=http%3A%2F%2Fwww.rasch.org%2Frmt%2Frmt94n.htm>
> and http://www.rasch.org/rmt/rmt163f.htm
> <https://api.thetopinbox.com/track/v2/link?id=00043403-6ad8-4f39-86a8-e0673884d97d&url=http%3A%2F%2Fwww.rasch.org%2Frmt%2Frmt163f.htm>
> - they both depend on the sample of persons, and expect the sample to have
> a normal distribution.
> Ben Wright's method at http://www.rasch.org/rmt/rmt144k.htm
> <https://api.thetopinbox.com/track/v2/link?id=00043403-6ad8-4f39-86a8-e0673884d97d&url=http%3A%2F%2Fwww.rasch.org%2Frmt%2Frmt144k.htm>
> is independent of the sample of persons. It is not G or H.
> Ben Wright suggests that if you need G or H for a skewed sample, then
> resample a normal distribution of persons from the skewed sample. See
> "Later Note" at the bottom of http://www.rasch.org/rmt/rmt144k.htm
> <https://api.thetopinbox.com/track/v2/link?id=00043403-6ad8-4f39-86a8-e0673884d97d&url=http%3A%2F%2Fwww.rasch.org%2Frmt%2Frmt144k.htm>
> OK?
> Mike L.
> On 3/1/2017 6:07 PM, Fabio La Porta wrote:
>> Hello Mike,
>> I understand that 1) is the formula for the separation ratio (G), but I
>> do not understand the formulas for 2) and 3).
>> Are these related to the calculation of the reliability index starting
>> from H [(4G+1)/3 ?] and, respectively, the method suggested by Ben Wright
>> for skewed distributions in http://www.rasch.org/rmt/rmt144k.htm
>> <https://api.thetopinbox.com/track/v2/link?id=00043403-6ad8-4f39-86a8-e0673884d97d&url=http%3A%2F%2Fwww.rasch.org%2Frmt%2Frmt144k.htm>
>> ?
>> Also, the above method for skewed distributions allows to calculate G or
>> H?
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