Year	Percent
1972	53.55
1973	54.66
1974	55.21
1975	56.24
1976	57.35
1977	58.44
1978	59.49
1979	61.16
1980	62.26
1981	62.69
1982	63.87
1983	65.3
1984	65.38
1985	65.64
1986	65.87
1987	66.65
1988	67.5
1989	68.33
1990	69.16
1991	70.75
1992	71.21
1993	71.61
1994	71.6
1995	71.29
1996	71.49
1997	71.36
1998	72.41
1999	73.1
2000	73.86
2001	74.11
2002	74.3
2003	74.41
2004	75.58
2005	75.81
2006	75.89
2007	75.91
2008	74.25
2009	77.04
2010	77.71
2011	77.32
2012	77.43
2013	77.25
2014	77.08
2015	77.69
2016	78.19
2017	78.33