Year	Percent
1971	86.14
1972	85.55
1973	84.37
1974	84.25
1975	83.33
1976	81.85
1977	80.02
1978	78.9
1979	78.38
1980	78.12
1981	77.82
1982	76.96
1983	77.72
1984	79.26
1985	79.39
1986	80.56
1987	80.71
1988	80.12
1989	79.74
1990	81.45
1991	79.68
1992	78.62
1993	79.93
1994	81.7
1995	82.49
1996	81.95
1997	80.59
1998	81.11
1999	81.25
2000	79.73
2001	78.6
2002	78.99
2003	79.21
2004	79.46
2005	78.49
2006	79.52
2007	80.02
2008	80.04
2009	80.94
2010	81.13