Year	Percent
1970	52
1971	53.1
1972	54.13
1973	54.61
1974	55.05
1975	56.05
1976	56.81
1977	57.03
1978	57.42
1979	57.91
1980	58.93
1981	60.65
1982	61.84
1983	61.14
1984	61.78
1985	61.94
1986	62.87
1987	64.51
1988	64.53
1989	64.73
1990	65.08
1991	65.87
1992	67.59
1993	68.14
1994	69.74
1995	70.19
1996	70.3
1997	71.04
1998	71.4
1999	72.43
2000	73.11
2001	73.75
2002	74.54
2003	75.28
2004	76.22
2005	76.2
2006	76.37
2007	76.19
2008	76.66
2009	78.75
2010	79
2011	79.05
2012	78.97
2013	79.34
2014	79.1
2015	79.66
2016	79.99
2017	80.14