Year	Percent
1971	85.2
1972	85.98
1973	88.79
1974	87.09
1975	87.1
1976	86.36
1977	85.4
1978	84.92
1979	84.35
1980	83.88
1981	83.75
1982	84.18
1983	84.84
1984	84.04
1985	84.69
1986	84.29
1987	88.83
1988	89.24
1989	89.51
1990	89.84
1991	90.5
1992	90.76
1993	89.55
1994	85.9
1995	83.28
1996	82.46
1997	83.39
1998	82.23
1999	81.13
2000	78.12
2001	80.82
2002	81.08
2003	77.81
2004	78.31
2005	77.37
2006	78.78
2007	77.91
2008	82.77
2009	82.48
2010	80.68