Year	Percent
1960	72.24
1961	70.96
1962	68.72
1963	66.64
1964	65.78
1965	63.64
1966	60.91
1967	59.89
1968	59.63
1969	58.93
1970	56.72
1971	54.43
1972	52.74
1973	50.74
1974	50.38
1975	47.23
1976	45.9
1977	43.84
1978	44.07
1979	45.8
1980	46.39
1981	45.37
1982	45.3
1983	44.99
1984	43.34
1985	43.66
1986	42.8
1987	40.75
1988	40.02
1989	39.34
1990	36.27
1991	36.32
1992	36.95
1993	34.21
1994	34.3
1995	33.71
1996	31.93
1997	32.38
1998	31.65
1999	32.84
2000	30.42
2001	31.31
2002	29.1
2003	28.87
2004	28.62
2005	29.51
2006	27.45
2007	27.1
2008	30.39
2009	29.25
2010	28.93