Alistair Cockburn has a very interesting paper on "The Costs and Benefits of Pair Programming". Of course Pair Programming is not the only "extreme" aspect of extreme programming but Alistair’s article contains some very interesting metrics (seems a lot less "extreme" after reading Alistair’s article). members.aol.com/humansandt/papers/pairprogrammingcostbene/pairprogrammingcostbene.htm