Year	Percent
1960	32.4
1961	30.88
1962	32.18
1963	33.76
1964	29.88
1965	29.93
1966	28.73
1967	31.53
1968	33.22
1969	33.05
1970	36.39
1971	34.81
1972	33.73
1973	34.42
1974	32.12
1975	32.4
1976	33.1
1977	31.87
1978	31.72
1979	32.23
1980	27.35
1981	29.3
1982	30.75
1983	30.08
1984	31.05
1985	31.12
1986	28.27
1987	29.07
1988	28.86
1989	27.87
1990	29.61
1991	30.61
1992	29.66
1993	29.84
1994	28.92
1995	29.68
1996	30.08
1997	27.55
1998	27.64
1999	27.78
2000	26.86
2001	26.68
2002	25.52
2003	24.65
2004	23.76
2005	24.6
2006	23.31
2007	22.56
2008	23.38
2009	23.89
2010	24.94