Data analysis using the Rasch model
We assumed that the 28-item EORTC QLQ-C30 fits
Rasch model's specification and forms a unidimensional
construct. If any item could not fit the Rasch model
expectation, patients were deemed to have exhibited
unexpected behaviors, such as aberrant responses, guessing
or inattentiveness, which may have led to responses
outside of the model's expectations (as we assumed all 28
items constructed a one-dimensional latent trait).
When the data fit the model's expectations, the infit
and outfit mean square error (MNSQ) statistics had an
expected value of unity on the items. The values of the
MNSQ statistics show the amount of distortion of the
measurement system. Values less than unity (or over-fit)
showed that the items were too predictable (i.e., there was
redundancy). Values greater than unity (or under-fit)
indicated unpredictability (i.e., noise). The MNSQ statistics
were chi-square statistics divided by their degrees of
freedom. Items with infit MNSQ beyond the range of 0.5-
1.5 were usually regarded as misfitting or poor-fitting
[13,14].