[Rasch] Differential Item Functioning: Help Needed

Mike Linacre rmt at rasch.org
Wed Feb 6 17:05:13 EST 2013


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?

Mike L.

DIF Nomogram is at http://www.winsteps.com/a/difnomogram.png

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