Year	Percent
1978	47.35
1979	48.16
1980	48.4
1981	50.17
1982	52.12
1983	53.81
1984	53.67
1985	54
1986	55.01
1987	55.54
1988	57.62
1989	60.09
1990	62.4
1991	64.31
1992	66.36
1993	69.55
1994	71.45
1995	72.43
1996	74.05
1997	75.47
1998	77.15
1999	78.57
2000	79.37
2001	80.42
2002	81.44
2003	82.66
2004	84.16
2005	84.66
2006	85.03
2007	85.69
2008	86.29
2009	79.83
2010	80.06
2011	80.01
2012	79.49
2013	79.32
2014	79.39
2015	79.39
2016	87.53
2017	87.78