Board Thread:Wiki discussions and announcements/@comment-4022742-20140519231344/@comment-1330314-20140520025403

Your reasoning actually follows a fairly logical progression, in which you're broadening the selection criterion each time. Each set you bring up is a subset of the next, which basically brings us to the helpful conclusion that the selection criterion needs to be as narrow as possible. Again, this leaves a lot of room for subjectivity, but at least it allows us to rule out the less parsimonious options on an item's "similar items" list. Effectively, we can apply this to every current item's list by trying to find a smaller list of items with a common theme. If we can, then this new list should replace the old, and we should remove all the options that don't fit in.

So now we have a trimming algorithm!