Year	Percent
1970	41.58
1971	42.91
1973	49.72
1974	50.28
1975	51.5
1977	50.54
1978	50.15
1979	50.57
1980	52.64
1981	50.36
1982	51.93
1983	53.61
1984	55.14
1985	56.12
1986	59.54
1987	59.25
1988	59.19
1989	58.98
1990	60.47
1991	59.38
1992	61.42
1993	62.25
1994	62.18
1995	63.82
1996	64.88
1997	64.09
1998	63.61
1999	64.08
2000	64.6
2001	63.1
2002	64.44
2003	65.29
2004	64.82
2005	64.25
2006	64.57
2007	64.31
2008	63.82
2011	67.46
2012	67.56
2013	67.36
2014	67.84
2015	68.39
2016	69.2