Little has changed in world purchasing power since 2003, with the exception of perennially expensive U.S. cities like New York and Chicago falling from the top, largely as a result of a weaker dollar, according to Swiss banking giant UBS. Using the Big Mac index, the authors found it takes an average of 35 minutes of work to buy the burger globally, but disparities are huge: ** In Nairobi, 1 1/2 hours of work are needed to buy the burger with the net hourly wage there. ** In the U.S. cities of Los Angeles, New York, Chicago and Miami, a...