Year	Percent
1960	23.03
1961	22.64
1962	24.41
1963	24.9
1964	23.97
1965	24.89
1966	25.15
1967	24.29
1968	24.69
1969	24.58
1970	31.4
1971	31.23
1972	31.5
1973	30.85
1974	26.88
1975	29.26
1976	30.09
1977	29.98
1978	29.83
1979	29.45
1980	28.18
1981	29.14
1982	27.71
1983	26.06
1984	28.68
1985	30.46
1986	24.55
1987	23.83
1988	22.2
1989	21.39
1990	20.37
1991	22.24
1992	22.94
1993	23.22
1994	22.94
1995	21.6
1996	22.22
1997	21.92
1998	21.7
1999	20.84
2000	20.66
2001	21.37
2002	20.64
2003	21.13
2004	20.55
2005	21.14
2006	20.46
2007	19.56
2008	19.96
2009	21.8
2010	21.45