# [Rasch] Fwd: Reliability, separation and strata

Fabio La Porta fabiolaporta at mail.com
Wed Mar 1 19:07:52 AEDT 2017

```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?

Also, the above method for skewed distributions allows to calculate G or H?

Fabio

2017-03-01 0:22 GMT+01:00 Mike Linacre <mike at winsteps.com>:

> Fabio:
>
> There are three different Reliability/Separation/Strata options:
>
> 1. Spearman "Test" Reliability for the current sample -> Cronbach Alpha in
> CTT, and "Separation Reliability" for the observed person sample in Rasch.
>       Separation ratio = square-root (Spearman Reliabity / (1-Spearman
> Reliability)). Normal distributions are assumed.
>
> 2. Strata for the current sample. This is the separation ratio, but with
> the tales of the observed person distribution treated as performance
> levels. Then "Strata reliability" = Strata^2 / ( 1+ Strata^2)
>
> 3. Wright's sample-independent method for strata. This shows the maximum
> number of statistically different strata the test can identify.
>      Then "Wright Strata Reliability" = Wright Strata^2 / ( 1+ Wright
> Strata^2). This is the maximum possible value the "test" reliability.
>
> Choose the one (and make clear which one you have chosen) that makes most
>
> OK?
>
> Mike L.
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>