Year	Percent
1970	37.27599
1971	37.76224
1972	37.88396
1973	38.33333
1974	38.63636
1975	39.04762
1976	39.31889
1977	39.57704
1978	39.82301
1979	40.05764
1980	40.44944
1981	40.93407
1982	41.28686
1983	41.62304
1984	41.94373
1985	42.25
1986	42.43902
1987	42.95943
1988	43.35664
1989	43.73576
1990	44.098
1991	44.34783
1992	44.89362
1993	45.32225
1994	45.73171
1995	46.12326
1996	46.49805
1997	46.95817
1998	47.39777
1999	47.90528
2000	48.3066
2001	48.86562
2002	49.14676
2003	49.66555
2004	50.16393
2005	50.72231
2006	51.1811
2007	51.69753
2008	52.19365
2009	52.67062
2010	53.207
2011	53.64807
2012	54.21348
2013	54.75862
2014	55.20974
2015	55.71809