Year	Percent
1970	37.03704
1971	37.41007
1972	37.5
1973	37.83784
1974	38.56209
1975	38.60759
1976	38.65031
1977	39.28571
1978	39.65517
1979	39.44444
1980	39.78495
1981	40.10417
1982	40.90909
1983	40.97561
1984	41.50943
1985	41.55251
1986	42.0354
1987	42.30769
1988	42.56198
1989	43.2
1990	43.41085
1991	43.82022
1992	44.2029
1993	44.5614
1994	44.89796
1995	45.21452
1996	45.80645
1997	46.0815
1998	46.48318
1999	46.86567
2000	47.23032
2001	47.44318
2002	48.05556
2003	48.23848
2004	48.67725
2005	49.09561
2006	49.49495
2007	49.87654
2008	50.12048
2009	50.70755
2010	51.15207
2011	51.57658
2012	51.98238
2013	52.37069
2014	52.74262
2015	53.19588