Year	Percent
1976	47.53
1977	46.32
1978	43.55
1979	43.87
1980	44.68
1981	46.09
1982	47.37
1983	48.72
1984	49.42
1985	50.86
1986	52.03
1987	53.03
1988	53.91
1989	55.16
1990	56.34
1991	57.17
1992	58.52
1993	60.22
1994	62.01
1995	62.96
2000	70.83
2001	71.47
2002	71.56
2003	71.94
2004	71.52
2005	71.21
2006	73.09
2007	73.1
2008	72.72
2009	74.25
2010	75.8
2011	75.27
2012	76.92
2013	79.33
2014	79.05
2015	79.78
2016	79.31
2017	80.43