Year	Percent
1974	39.85
1975	45.13
1976	43.37
1977	45.1
1978	46.8
1979	45.97
1980	45.39
1981	46.16
1982	47.48
1983	48.7
1984	49.29
1985	49.73
1986	50.52
1987	51.42
1988	52.16
1989	52.76
1990	53.72
1991	54.44
1992	55.97
1993	56.32
1995	52.81
1996	53.41
1997	53.88
1998	54.08
1999	54.95
2001	58.3
2002	58.78
2003	58.27
2004	58.75
2005	60.19
2006	60.72
2007	61.8
2008	62.69
2009	64.8
2010	65.43
2015	67.11
2016	66.88