Year	Percent
1960	52.39
1961	52.36
1962	48.35
1963	49.21
1964	45.02
1965	43.23
1966	44.45
1967	40.64
1968	43.64
1969	41.72
1970	39.43
1971	35.89
1972	35.5
1973	35.31
1974	36.25
1975	36.29
1976	36.16
1977	33.32
1978	29.92
1979	30.25
1980	28.75
1981	29.69
1982	32.03
1983	33.02
1984	30.39
1985	29.87
1986	31.07
1987	29.32
1988	29.37
1989	28.75
1990	29.33
1991	28.63
1992	26.74
1993	26.71
1994	25.65
1995	25.74
1996	25.79
1997	24.24
1998	24.34
1999	23.67
2000	22.96
2001	23.33
2002	23.25
2003	24.26
2004	24.26
2005	23.72
2006	23.64
2007	23.09
2008	24.93
2009	27.06
2010	28.29