Determining the importance of items across sets?

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There's a better term for it. Let's say i have a suite of sets - A, B, C, D.

Each set contains any number, in a non-important order, of items - 1,2,3,4,5,6 The items may occur any number (0 or more) of times in a set.

Is there some agreed-upon formula to determine the "most important" item? Or are there algorithms for this?