Year	Percent
1971	48.58
1972	49.43
1973	48.38
1974	52.87
1975	51.08
1976	59.22
1977	58.68
1978	58.4
1979	57.52
1980	60.85
1981	61.41
1982	60.87
1983	59.5
1984	60
1985	60.56
1986	59.52
1987	58.25
1988	59.92
1989	61.27
1990	59.34
1991	59.52
1992	57.89
1993	55.72
1994	53.86
1995	51.42
1996	49.98
1997	49.27
1998	49.06
1999	48.63
2000	48.15
2001	46.8
2002	44.64
2003	44.44
2004	41.48
2005	40.44
2006	40.13
2007	37.92
2008	36.2
2009	34.49
2010	33.02