[Rasch] sample size

iasonas liasonas at cytanet.com.cy
Wed Feb 24 17:39:39 EST 2010

 Dear all,
I have an MSc student who collectd data on two groups of students. Each
group consists of 20 students. Each student was measured on three different
essays. Each essay was evaluated twice: once when the essay was in a draft
state, and once when it was finished. Each time an essay was evaluated, it
was evaluated by three persons: the teacher, an external teacher and a
student (peer-evaluation). This is a great opportunity of Many-Facets Rasch
model. I have many years of intense experience with Rasch models, and I do
NOT worry about the sample size; I am sure that the parameters will be
estimated with a realistic/practical precision (provided the data fit the
model - we havent yet finished data collection). However, would you also all
agree that I should not worry about the sample size? The student is
reluctant to collect more data for a number of reasons. Should I give her
the green light to go on to collect data from only 20 students per group, or
does any one of you worry that the data will be too "thin"?

Thank you

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