[Rasch] Differential Item Functioning: Help Needed
svkr at sund.ku.dk
Wed Feb 6 22:28:36 EST 2013
Hard to say Mike, we need a little more information, e.g.:
What kind of DIF test are you using? Different test may have very different power and may therefore require different sample sizes.
How many items are you using in addition to the DIF item? The number of items has a strong effect on the error of person estimates and therefor also on the power of tests using these estimates (like, for instance, the DIF test in RUMM). Are you assuming that the number of items is infinite so that we have person estimates without error and bias.
What do you mean by "DIF at 80%"?
PS it looks to me as if you need about 2 times 500 persons to disclose DIF equal to 0.25. Is that right? And if that is correct we can say good bye to small samples.
From: rasch-bounces at acer.edu.au [rasch-bounces at acer.edu.au] on behalf of Mike Linacre [rmt at rasch.org]
Sent: Wednesday, February 06, 2013 7:05 AM
To: rasch at acer.edu.au
Subject: [Rasch] Differential Item Functioning: Help Needed
Here is a DIF nomogram showing the Reference and Focal group sizes to
obtain p=.05 statistical significance with various logit sizes of DIF on
dichotomous items (MCQs, etc.). For example, the nomogram indicates we
need groups of around 100 to detect DIF of 1 logit, and groups of about
1500 (log-scaled) to detect DIF of 0.25 logits.
It is based on statistical theory, but with approximations and assumptions.
Does it match your experience? Are the indicated group sizes too small,
too large or about right?
DIF Nomogram is at http://www.winsteps.com/a/difnomogram.png
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