Year	Percent
1971	85.2
1972	84.49
1973	84.01
1974	82.53
1975	80.68
1976	80.45
1977	77.84
1978	74.83
1979	72.83
1980	70.13
1981	68.99
1982	68.61
1983	68.41
1984	68.7
1985	65.32
1986	62.93
1987	61.36
1988	59.87
1989	59.38
1990	52.15
1991	51.24
1992	50.45
1993	48.88
1994	47.25
1995	45.94
1996	44.04
1997	43.69
1998	45.56
1999	43.85
2000	43.16
2001	42.95
2002	43.35
2003	42.68
2004	41.87
2005	41.54
2006	39.98
2007	40.6
2008	40.25
2009	38.48
2010	37.03