Year	Percent
1971	77.88
1972	78.62
1973	77.95
1974	78.22
1975	79.57
1976	79.37
1977	79.77
1978	81.47
1979	82.08
1980	80.62
1981	81.89
1982	82.19
1983	82.43
1984	83.13
1985	82.83
1986	82.75
1987	82.62
1988	82.6
1989	82.64
1990	82.86
1991	83.34
1992	82.31
1993	81.29
1994	82.35
1995	82.04
1996	83.09
1997	82.91
1998	82.2
1999	82.1
2000	80.74
2001	77.55
2002	77.42
2003	74.31
2004	72.89
2005	73.54
2006	73.17
2007	72.2
2008	72.62
2009	73.06
2010	72.59