Year	Percent
1975	54.89
1976	55.03
1977	55.2
1978	55.3
1979	56.66
1980	57.28
1981	57.79
1982	57.55
1983	59.37
1984	59.11
1985	58.87
1986	58.24
1987	58.1
1989	60.63
1990	61.22
1991	61.29
1992	61.55
1993	62.4
1994	61.99
1995	62.84
1996	63.98
1997	65.14
1998	65.34
1999	67.04
2000	66.49
2001	68.18
2002	69.16
2003	69.15
2004	68.22
2005	68.63
2006	68.02
2007	67.49
2008	68.44
2009	68.7
2010	69.16
2011	70
2012	70.75
2013	71.06