Year	Percent
1960	38.95
1961	40.74
1962	39.61
1963	40.82
1964	42.23
1965	41.32
1966	42.53
1967	45.92
1968	45.7
1969	50.37
1970	47.92
1971	46.16
1972	44.9
1973	43.47
1974	42.56
1975	44.55
1976	43.42
1977	42.9
1978	41.63
1979	43
1980	41.68
1981	43.33
1982	42.53
1983	41.73
1984	39.51
1985	39.39
1986	38.29
1987	37.88
1988	38.45
1989	38.37
1990	35.19
1991	34.5
1992	34.82
1993	35.22
1994	32.39
1995	33.31
1996	30.64
1997	31.01
1998	28.76
1999	29.08
2000	28.57
2001	28.99
2002	29.34
2003	29.26
2004	27.51
2005	27.51
2006	26.23
2007	23.41
2008	17.12
2009	17.61
2010	17.29