Year	Percent
1971	62.86
1972	63.15
1973	62.21
1974	62.49
1975	61.5
1976	62.25
1977	62.69
1978	61.43
1979	60.7
1980	60.06
1981	58.36
1982	56.75
1983	57.13
1984	56.85
1985	56.37
1986	56.37
1987	55.43
1988	54.77
1989	54.95
1990	55.37
1991	55.63
1992	53.97
1993	52.67
1994	52.97
1995	52.79
1996	51.99
1997	52.69
1998	52.02
1999	51.43
2000	52.48
2001	52.54
2002	52.03
2003	50.23
2004	48.9
2005	48.41
2006	47.97
2007	46.41
2008	48.06
2009	47.85
2010	48.31