Year	Percent
1980	36.83
1981	35.46
1982	35.52
1983	36.1
1984	36.78
1985	37.56
1986	38.53
1987	39.34
1988	40.07
1989	40.98
1990	45.01
1991	47.8
1992	53.57
1993	57.07
1994	58.27
1995	59.4
1996	58.53
1997	58.96
1998	57.88
1999	58.69
2000	59.76
2001	59.39
2002	59.74
2003	61.22
2004	61.79
2005	62.66
2006	62.92
2007	62.9
2008	63.46
2009	64.23
2010	64.76
2011	64.28
2012	65.14
2013	65.33
2014	64.57
2015	64.67
2016	64.47
2017	63.44