Year	Percent
1974	30.4
1975	32
1976	32.7
1977	34.22
1978	33.82
1979	34.77
1980	36.1
1981	37.13
1982	37.28
1983	40.99
1984	42.41
1985	42.42
1986	44.68
1987	43.87
1988	44.23
1989	46.14
1990	47.83
1991	48.64
1992	56.1
1993	55.58
1994	55.76
1995	56.34
1996	56.52
1997	55.69
1998	50.86
1999	52.86
2000	52.88
2001	53.38
2002	54.04
2003	54.37
2004	56.64
2005	57.56
2006	57.78
2007	57.92
2008	59.57
2009	60.75
2010	61.55
2011	62.94
2012	64.04
2013	66.07
2014	67.49
2015	68.12
2016	68.59
2017	68.86