Year	Percent
1960	34.32
1961	34.46
1962	35.72
1963	38.33
1964	34.92
1965	33.98
1966	33.41
1967	31.22
1968	31.98
1969	33.15
1970	31.54
1971	28.57
1972	28.08
1973	27.41
1974	25.02
1975	27.09
1976	25.69
1977	25.91
1978	25.03
1979	24.38
1980	26.09
1981	24.91
1982	26.79
1983	24.95
1984	22.32
1985	23.77
1986	20.58
1987	20.74
1988	19.89
1989	19.86
1990	20.25
1991	20.91
1992	19.87
1993	20.66
1994	20.32
1995	20.26
1996	21.97
1997	19.69
1998	19.02
1999	18.85
2000	18.16
2001	18.46
2002	17.88
2003	17.72
2004	17.48
2005	16.8
2006	17.45
2007	14.75
2008	15.68
2009	16.95
2010	17.78