val urban_prices = List(List(Some(71.539941), None), 
                        List(Some(76.974614), None), 
                        List(Some(65.226685), Some(6.35)), 
                        List(Some(78.354649), Some(12.241)), 
                        List(Some(85.517645), Some(38.188)))

val urban_deltas = List(List(Some(0.07596697626574789), None), 
                        List(Some(-0.152620823795232), None), 
                        List(Some(0.20126676681483952), Some(0.9277165354330709)), 
                        List(Some(0.09141762603007778), Some(2.119679764725104)))


assert (get_deltas(get_prices(List("IBM", "BIDU"), 2004 to 2008)) == urban_deltas)