This collusion detection (test cheating) index simply calculates the number of responses in common between a given pair of examinees. For example, both answered ‘B’ to a certain item regardless of whether it was correct or incorrect. There is no probabilistic evaluation that can be used to flag examinees. However, it could be of good use from a descriptive or investigative perspective.
It has a major flaw in that we expect it to be very high for high-ability examinees. If two smart examinees both get 99/100 correct, the minimum RIC they could have is 98/100. Even if they have never met each other and have no possibility of collusion or cheating.
Note that RIC 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 90 items. But for a 50-item test, this is obviously irrelevant, and you might want to set it at 45.
Problems such as these with Responses In Common have led to the development of much more sophisticated indices of examinee collusion and copying, such as Holland’s K index and variants.
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