Year	Percent
1970	45.54
1971	45.96
1972	46.75
1973	47.6
1974	48.3
1975	50.13
1976	51.34
1977	52.01
1978	52.55
1979	54.72
1980	55
1981	55.58
1982	56.63
1983	57.61
1984	58.09
1985	58.38
1986	59.04
1987	60.28
1988	61.52
1989	62.47
1990	63.57
1991	66.53
1992	68.21
1993	68.69
1994	68.69
1995	67.64
1996	66.44
1997	67.53
1998	67.27
1999	67.9
2000	67.47
2001	70.86
2002	71.4
2003	72.73
2004	73.3
2005	73.23
2006	73.09
2007	72.95
2008	67.87
2009	70.58
2010	70.48
2011	70.99
2012	72
2013	72.09
2014	72.76
2015	73.78
2016	74.07
2017	74.55