Here are several ways to think about this issue:
Now, the fact that the vignettes are subject to DIF and are interpreted in different ways in different cultures by different people is not a problem in and of itself. In fact, the technique relies on vignettes having DIF too. What would be a problem is if the nature of the DIF differs for the vignettes and the self-assessments. In that case, suppose we have DIFv for the vignettes but DIFs for the self-assessments, and so our correction would be DIFsADIFv , which is the same as DIFsDIFvA . So the ultimate question is not whether the vignettes have DIF, but rather whether DIFsDIFv or DIFs is closer to zero. For precisely the reason that it is the same person with the same biases answering both questions, DIFsDIFv will normally be closer to zero than DIFs. This is the reason why we find that this technique usually is an improvement over self-assessments alone and why only in rare situations does the correction make things worse.