Year	Percent
1972	55.53
1973	56.18
1974	57.49
1975	58.25
1976	58.66
1977	59.71
1978	60.12
1979	59.28
1981	62.54
1983	64.32
1984	66.3
1985	65.24
1986	63.45
1987	63.71
1988	66.73
1989	66.45
1990	66.67
1991	66.27
1992	67.6
1993	68.46
1994	68.13
1995	68.41
1996	69.62
1997	69.73
1998	69.66
1999	69.5
2000	70.76
2001	70.91
2002	73.12
2003	73.41
2004	72.88
2005	72.81
2006	73.49
2007	73.45
2008	74.36
2009	77.08
2010	77.93
2011	77.65
2012	77.6
2013	77.83
2014	78.06
2015	77.97
2016	78.06
2017	78.22