Year	Percent
1971	51.9
1972	52.05
1973	49.97
1974	47.61
1975	47.97
1976	45.45
1977	44.04
1978	46.55
1979	54.4
1980	51.35
1981	50.15
1982	50.57
1983	51.75
1984	52.38
1985	51.71
1986	53.94
1987	54.04
1988	57.5
1989	59.85
1990	58.32
1991	61.14
1992	61.86
1993	62.43
1994	59.92
1995	59.95
1996	58.96
1997	57.39
1998	58.85
1999	55.06
2000	55.7
2001	57.52
2002	56.22
2003	59.98
2004	60.38
2005	60.06
2006	56.18
2007	47.45
2008	49.19
2009	50.88
2010	48.61