Year	Percent
1971	75.15
1972	74.8
1973	74.97
1974	74.93
1975	76.1
1976	75.35
1977	73.77
1978	74.69
1979	75.59
1980	75.45
1981	75.73
1982	77.51
1983	78.75
1984	76.82
1985	76.64
1986	76.17
1987	76.17
1988	77.38
1989	77.32
1990	77.49
1991	78.46
1992	78.95
1993	79.44
1994	80.11
1995	79.94
1996	79.34
1997	80.46
1998	82.48
1999	83.87
2000	84.53
2001	82.86
2002	82.56
2003	83.4
2004	83.3
2005	82.19
2006	79.98
2007	80.15
2008	80.67
2009	78.89
2010	76.48