Year	Percent
1983	70.95
1984	70.69
1985	71.05
1986	71.35
1987	71.83
1988	72.09
1989	72.32
1990	72.69
1991	73.52
1992	74.12
1993	74.72
1994	74.71
1995	74.68
1996	74.86
1997	74.87
1998	75.37
1999	75.74
2000	73.96
2001	74.75
2002	75.71
2003	76.05
2004	76.16
2005	76.31
2006	76.26
2007	76.57
2008	77.2
2009	78.61
2010	78.92
2011	78.81
2012	78.88
2013	78.72
2014	78.66
2015	78.7
2016	78.78
2017	78.84