Year	Percent
1960	21.36665
1961	21.87318
1962	22.39869
1963	22.94337
1964	23.51765
1965	24.12313
1966	24.76765
1967	25.45
1968	26.17432
1969	26.93651
1970	27.74407
1971	28.31672
1972	28.86537
1973	29.43796
1974	30.03544
1975	30.65345
1976	31.29493
1977	31.95665
1978	32.64532
1979	33.36034
1980	34.10609
1981	34.28533
1982	34.35765
1983	34.44835
1984	34.56747
1985	34.71728
1986	34.90295
1987	35.12155
1988	35.37293
1989	35.65073
1990	35.95673
1991	36.2908
1992	36.50324
1993	36.70177
1994	36.91475
1995	37.13762
1996	37.37021
1997	37.6115
1998	37.86981
1999	38.15071
2000	38.4604
2001	38.52038
2002	38.57758
2003	38.65741
2004	38.76125
2005	38.8867
2006	39.03703
2007	39.21065
2008	39.40472
2009	39.61045
2010	39.82641
2011	40.01459
2012	40.20894
2013	40.41246
2014	40.63076
2015	40.86543
2016	41.11776
2017	41.38471