Year	Percent
1971	59.39
1972	59.39
1973	56.49
1974	59.94
1975	57.96
1976	58
1977	59.6
1978	60.28
1979	60.61
1980	57.65
1981	57.15
1982	58.27
1983	58.32
1984	60.62
1985	60.65
1986	60.51
1987	60.2
1988	60.92
1989	61.62
1990	57.97
1991	58.77
1992	59.37
1993	60.39
1994	63.74
1995	61.95
1996	61.51
1997	62.19
1998	62.73
1999	58.73
2000	63.59
2001	68.3
2002	68.62
2003	68.26
2004	73.08
2005	71.31
2006	71.93
2007	72.16
2008	74.25
2009	73.86
2010	73.23