Year	Percent
1960	36.67
1961	35.6
1962	36
1963	35.55
1964	32.65
1965	31.22
1966	29.06
1967	26.66
1968	25.81
1969	25.55
1970	25.14
1971	24.22
1972	24.03
1973	23.51
1974	24.86
1975	25.08
1976	24.55
1977	24.81
1978	25.18
1979	26
1980	27.36
1981	27.78
1982	27.61
1983	27.61
1984	26.92
1985	28.55
1986	28.9
1987	28.5
1988	27.18
1989	26.47
1990	27.05
1991	28.41
1992	28.37
1993	28.95
1994	27.93
1995	27.34
1996	29.19
1997	27.88
1998	28.56
1999	28.09
2000	28.59
2001	29.39
2002	29.33
2003	29.81
2004	30.3
2005	29.71
2006	29.5
2007	29.23
2008	29.61
2009	30.21
2010	32.33