Year	Percent
1970	42.14
1971	43.28
1972	43.65
1974	47.11
1975	46.88
1976	47.16
1977	47.93
1978	50.95
1979	49.94
1982	51.19
1983	51.72
1984	52.73
1985	50.7
1986	52.38
1987	56.82
1988	55.94
1989	56.23
1991	58.2
1992	58.01
1993	59.29
1994	61.63
1995	61.13
1996	61.47
1997	62.99
1998	64.17
1999	64.37
2000	65.67
2001	64.62
2002	65.65
2003	65.3
2004	66.34
2005	67.14
2006	65.98
2007	65.98
2008	63.01
2009	62.79
2010	63.88
2011	64.69
2012	65.03
2013	63.5
2014	64.21
2015	66.61
2016	66.25
2017	66.96