Year	Percent
1960	33.29
1961	34.98
1962	38.11
1963	41.06
1964	35.44
1965	35.63
1966	35.46
1967	35.94
1968	35.91
1969	33.92
1970	33.48
1971	31.92
1972	32.98
1973	32.75
1974	32.31
1975	33.41
1976	26.62
1977	27.05
1978	28.49
1979	29.62
1980	29.69
1981	29.5
1982	27.88
1983	27.84
1984	27.82
1985	29.83
1986	29.87
1987	28.82
1988	25.98
1989	25.32
1990	25.81
1991	26.83
1992	26.37
1993	26.53
1994	23.98
1995	24.55
1996	26.29
1997	24.72
1998	24.47
1999	23.35
2000	22.89
2001	23.39
2002	23.25
2003	23.46
2004	23.95
2005	23.88
2006	21.89
2007	20.44
2008	20.72
2009	21.11
2010	21.51