Year	Percent
1970	33.34
1971	34.26
1972	38.04
1973	38.97
1974	39.72
1975	39.68
1976	41.33
1977	41.66
1978	42.23
1979	43.54
1980	44.73
1981	45.99
1982	47.39
1983	47.74
1984	48.68
1985	49.84
1986	51.87
1987	52.4
1988	53.16
1989	54.16
1990	54.58
1991	56.06
1992	57.48
1993	59.18
1994	60.25
1995	60.81
1996	62.17
1997	62.04
1998	61.79
1999	62.11
2000	62.48
2001	61.99
2002	62.96
2003	63.58
2004	64.31
2005	65.13
2006	65.79
2007	66.37
2008	68.12
2009	71.29
2010	72.83
2011	74.14
2012	75.11
2013	75.95
2014	76.31
2015	75.97
2016	76.17
2017	75.59