Year	Percent
1960	25.64
1961	24.47
1962	25.24
1963	25.59
1964	21.63
1965	20.08
1966	15.55
1967	17.51
1968	17.12
1969	17.78
1970	24.65
1971	24.64
1972	23.15
1973	22.98
1974	23.39
1975	23.51
1976	21.55
1977	20.09
1978	22.16
1979	19.67
1980	18.58
1981	18.93
1982	21.09
1983	19
1984	18.7
1985	19.37
1986	19.13
1987	20.73
1988	19.97
1989	19.78
1990	21.01
1991	21.15
1992	21.36
1993	21.32
1994	21.5
1995	21.63
1996	23.84
1997	23.94
1998	23.38
1999	23.75
2000	24.28
2001	24.6
2002	25.12
2003	26.67
2004	26.48
2005	26.44
2006	25.68
2007	24.66
2008	24.6
2009	23.55
2010	23.79