[Rasch] Fwd: Reliability, separation and strata
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.
More information about the Rasch