Year	Percent
1970	32.52033
1971	32.53968
1972	32.94574
1973	33.20755
1974	33.57934
1975	33.81295
1976	34.03509
1977	34.36426
1978	34.44816
1979	34.96732
1980	35.14377
1981	35.51402
1982	35.97561
1983	36.0119
1984	36.33721
1985	36.93182
1986	37.22222
1987	37.39837
1988	37.93103
1989	38.0829
1990	38.48101
1991	38.86139
1992	39.22518
1993	39.57346
1994	40.0463
1995	40.36281
1996	40.79823
1997	40.99783
1998	41.40127
1999	41.78794
2000	42.27642
2001	42.62948
2002	43.07992
2003	43.40344
2004	43.82022
2005	44.22018
2006	44.70377
2007	45.07042
2008	45.59585
2009	46.02369
2010	46.43449
2011	46.82927
2012	47.36842
2013	47.73083
2014	48.23349
2015	48.71795