Year	Percent
1971	77.31
1972	76.91
1973	76.68
1974	77.25
1975	77.5
1976	77.18
1977	77.57
1978	77.45
1979	77.24
1980	77.52
1981	77.6
1982	77.88
1983	77.9
1984	78.03
1985	77.85
1986	78.13
1987	78.11
1988	78.16
1989	78.17
1990	78.08
1991	78.15
1992	78.21
1993	78.04
1994	78.12
1995	77.55
1996	77.13
1997	78.45
1998	77.05
1999	78.09
2000	77.97
2001	77.11
2002	76.23
2003	75.94
2004	75.26
2005	74.2
2006	73.67
2007	73.23
2008	72.63
2009	72.88
2010	72.41