[Rasch] Fwd: Reliability, separation and strata

Mike Linacre mike at winsteps.com
Wed Mar 1 10:22:17 AEDT 2017


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 
       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 sense in your situation.


Mike L.

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