This exam cheating index (collusion detection) simply calculates the number of errors in common between a given pair of examinees. For example, two examinees got 80/100 correct, meaning 20 errors, and they answered all of the same questions wrongly, the EIC would be 20. If they both scored 80/100 but had only 10 wrong questions in common, the EIC would be 10. There is no probabilistic evaluation that can be used to flag examinees, as with more advanced indices. In fact, it is used inside some other indices, such as Harpp & Hogan. However, this index could be of good use from a descriptive or investigative perspective.
Note that EIC is not standardized in any way, so its range and relevant flag cutoff will depend on the number of items in your test, and how much your examinee responses vary. For a 100-item test, you might want to set the flag at 10 items. But for a 30-item test, this is obviously irrelevant, and you might want to set it at 5 (because most examinees will probably not even get more than 10 errors).
Learn more about applying EIC with SIFT, a free software program for exam cheating detection and other assessment issues.
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