Now why might you want to do this why might you want to know whether two different people ranked things in the similar way had similar preferences well one reason might be what's called collaborative filtering, probably many of you have had the experience of going to a website and if you've made a few purchases through this website it starts recommending further purchases for you, so one way to solve this problem under the hood, is to look at your purchases look at what you seem to like, find other people who have similar preferences similar history look at things they've bought that you haven't, and then recommend. 