Year	Percent
1960	45.17
1961	41.13
1962	43.73
1963	44.92
1964	50.24
1965	49.71
1966	46.9
1967	48.46
1968	48.03
1969	46.99
1970	45.29
1971	45.01
1972	43.58
1973	45.26
1974	44.91
1975	41.18
1976	41.45
1977	40.41
1978	39.02
1979	39.69
1980	37.85
1981	37.52
1982	31.16
1983	33.13
1984	32.03
1985	35.88
1986	35.01
1987	35.61
1988	33.13
1989	31.59
1990	30.39
1991	31.17
1992	30.42
1993	31.88
1994	30.78
1995	30.85
1996	31.68
1997	30.34
1998	30.45
1999	29.79
2000	29.24
2001	30.23
2002	29.95
2003	30.04
2004	29.65
2005	29.84
2006	29.48
2007	29.39
2008	29.72
2009	31.09
2010	32.74