Year	Percent
1971	48.59
1972	47.1
1973	46.08
1974	46.31
1975	46.42
1976	46.87
1977	49.67
1978	50.9
1979	52.6
1980	52.69
1981	53.93
1982	55.03
1983	55.88
1984	56.4
1985	57.31
1986	58.43
1987	58.9
1988	58.03
1989	58.75
1990	61.88
1991	61.54
1992	61.73
1993	63.72
1994	65.18
1995	65.96
1996	68.05
1997	67.16
1998	68.14
1999	69.48
2000	69.36
2001	69.01
2002	68.71
2003	68.33
2004	68.25
2005	67.86
2006	68.04
2007	71.13
2008	71.52
2009	72.13
2010	71.31