Year	Percent
1971	81.9
1972	75.13
1973	77.47
1974	78.79
1975	80.97
1976	81.91
1977	86.03
1978	77.44
1979	78.43
1980	77.45
1981	78.75
1982	81.2
1983	81.04
1984	80.14
1985	76.75
1986	77.63
1987	77.17
1988	75.64
1989	75.48
1990	71.43
1991	71.06
1992	71.78
1993	69.88
1994	71.86
1995	73.75
1996	73.15
1997	74.06
1998	75.51
1999	73.61
2000	73.52
2001	73.05
2002	71.69
2003	67.73
2004	62.98
2005	68.91
2006	65.84
2007	63.05
2008	60.7
2009	60.65
2010	57.01