Year	Percent
1975	52.58
1976	57.47
1977	57.25
1978	58.49
1979	58.92
1980	59.83
1981	60.67
1982	65.43
1983	66
1984	63.74
1985	59.58
1986	58.19
1987	56.04
1988	54.84
1989	54.13
1990	55.48
1991	54.59
1992	55.5
1993	56.21
1994	57.72
1995	58.22
1996	58
1997	58.28
1998	60.04
1999	62.14
2000	62.17
2001	62.47
2002	62.64
2003	63.01
2004	63.04
2005	63.89
2006	63.87
2007	64.3
2008	64.74
2009	33.14
2010	66.44
2011	66.36
2012	66.19
2013	66.65
2014	67.11
2015	67.34
2016	67.51
2017	68.01