[Rasch] fit analysis
Mike Linacre (RMT)
rmt at rasch.org
Mon Jun 5 13:05:30 EST 2006
You wrote: "N= 1054. Mean square fit statistics for items are good, but
the standardized stats for three items are bad (outfit standardized MNSQs
of 3.8, -2.9, -5.4)."
Reply: Thank you for being detailed ... we can tell you what your
mean-squares are from the plot on www.rasch.org/rmt/rmt171n.htm
t = 3.8, Mnsq = 1.2; t = -2.9, Mnsq = 0.9; t = -5.4, Mnsq = 0.8
Conclusion: your fit tests are too sensitive due to the large sample size.
You have turned the fit "magnification" up so high that you can see every
scratch on the pain of glass. The fit statistics tell you: "these data
aren't perfect". The mean-squares tell you "but they are good enough for
government work!" (As Ben Wright often used to say.)
The useful sample size range for the type of significance test you are
using is 30 to 300 (or so).
Editor, Rasch Measurement Transactions
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