[Rasch] item selection based on DIF

DIMA ALEXANDRA alexandra.dima at univ-lyon1.fr
Mon Oct 30 01:19:17 AEDT 2017

Dear Rasch researchers,

I would like to ask for your advice in a DIF-related issue. We performed a Rasch analysis of a 33-item questionnaire to choose items for a short version, in two steps. First we examined items infit and outfit, and used the thresholds of mean squares outside the 0.6-1.4 range and standardized fit statistics outside the +/-2.0. No items were excluded at this step (all fitted this criterion). Then we examined DIF for gender, age, and education (in this order), and excluded items with DIF > 0.5 logits. We excluded 3 items based on gender DIF, 4 items based on age DIF, and 12 based on education DIF. We thus arrived at a 14-item version, for which we examined again DIF for all 3 variables. We noticed that there appeared DIFs for age > 0.5 logits (e.g. .60, p<.000), even if in the earlier steps of the selection process these items had no problems. This is a noticeable and significant difference according to the winsteps manual (http://www.winsteps.com/winman/table30_1.htm ): « [DIF CONTRAST] should be at least 0.5 logits for DIF to be noticeable. "Prob." shows the probability of observing this amount of contrast by chance, when there is no systematic item bias effect. For statistically significance DIF on an item, Prob. ≤ .05. ». But I do not know how to figure out if this is also a substantive difference and if I should exclude those items as well, in other words continue item selection until all items show DIF <.05 for all variables (age, gender, education). This is particularly puzzling since these items showed acceptable DIF in previous runs. I understand in principle that this can happen, but what does it mean: are this items good enough or not? Should they be kept or excluded? Are there other criteria and tests that I should consider?

Any suggestions or references for further reading would be much appreciated!

Many thanks,

Alexandra DIMA PhD, AFBPsS
Marie Curie Research Fellow

Health Services and Performance Research
Université Claude Bernard Lyon 1
Domaine Rockefeller- 2eme étage (aile CD)
8 avenue Rockefeller
69373 Lyon 8
W: +33 (0) 4 26 68 82 23
M: +33 (0) 6 32 86 82 37
alexandra.dima at univ-lyon1.fr<mailto:Alexandra.dima at univ-lyon1.fr>

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