Year	Percent
1960	27.75
1961	27.36
1962	27.1
1963	26.33
1964	25.56
1965	24.9
1966	23.46
1967	22.71
1968	21.9
1969	23.28
1970	24.64
1971	24.08
1972	23.65
1973	22.78
1974	23.48
1975	23.6
1976	24.82
1977	23.55
1978	24.24
1979	24.6
1980	25.86
1981	27.14
1982	29.55
1983	28.06
1984	27.74
1985	29.01
1986	29.18
1987	30.58
1988	30.85
1989	30.82
1990	29.02
1991	33.17
1992	34.56
1993	37.61
1994	36.73
1995	35.1
1996	33.59
1997	32.29
1998	31.16
1999	31.86
2000	29.54
2001	32.23
2002	31.29
2003	30.28
2004	29.23
2005	29.81
2006	29.92
2007	28.15
2008	28.33
2009	28.94
2010	30.14