[Rasch] Differences in alpha for Winsteps and R

Stuart Luppescu lupp at uchicago.edu
Wed Jul 12 04:00:46 AEST 2017


Hello fellow Raschies, I'm writing reports on a number of assessments,
including Rasch and non-Rasch statistics. It's easier to calculate the
Cronbach's alpha using the psych package in R than to read it from the
Winsteps output so that's what I'm using for that. But I've noticed
that what psych::alpha() calculates is rather different from what
appears in Table 3 in Winsteps. For example, for 1 assessment, Winsteps
reports (for all forms together):
CRONBACH ALPHA (KR-20) PERSON RAW SCORE RELIABILITY = .86 (approximate
due to missing data)
but psych::alpha() gives 0.55 or 0.68 (depending on the form)

Anyone know why there should be such a difference?

TIA
-- 
Stuart Luppescu
Chief Psychometrician (ret.)
UChicago Consortium on School Research
http://consortium.uchicago.edu

lupp at uchicago.edu


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