9. The apparatus according to claim 8 , wherein the processor is configured to identify, in each of the second lists, a number of the search results that are stable.
11. The apparatus according to claim 9 , wherein the processor is configured to compute the stability score for a given first search result in the first list by computing, for each given corresponding second search result in each given second list, a first additional stability score based on a stability of the given second search result and the first ranking of the given first search result, computing a second additional stability score based on a difference between the second ranking of the given second search result and the first ranking of the given first result, and computing a third additional stability score based on the stability of the given-second search result in the given second list and the number of the stable second search results in the given second list.