Year	Percent
1971	98.3
1972	98.11
1973	97.25
1974	97.32
1975	97.75
1976	98.16
1977	97.88
1978	97.9
1979	96.88
1980	96.04
1981	96.4
1982	95.97
1983	95.06
1984	94.88
1985	96.21
1986	95.27
1987	95.25
1988	95.53
1989	96.28
1990	94.86
1991	94.19
1992	94.41
1993	93.97
1994	92.98
1995	92.54
1996	92.63
1997	92.21
1998	92.03
1999	89.78
2000	89.53
2001	89.48
2002	91.01
2003	90.74
2004	90.73
2005	89.81
2006	91.13
2007	90.93
2008	89.53
2009	87.96
2010	87.36