Year	Percent
1970	46.72
1971	47.94
1972	48.79
1973	49.15
1974	49.89
1975	51.31
1976	51.81
1977	52.56
1978	53.11
1979	53.71
1980	53.99
1981	54.51
1982	55.18
1983	55.7
1984	55.93
1985	55.97
1986	56.71
1987	57.54
1988	57.49
1989	57.7
1990	58.22
1991	58.39
1992	58.55
1993	59.33
1994	59.72
1995	60.35
1996	60.78
1997	61.06
1998	62.13
1999	62.52
2000	63.11
2001	63.94
2002	64.79
2003	65.14
2004	65.98
2005	66.85
2007	66.69
2008	67.69
2009	68.83
2010	69.5
2011	27.86
2012	69.23
2013	69.05
2014	69.99
2015	69.35
2016	69.78
2017	69.85