Year	Percent
1960	0.31
1961	0.45
1962	0.45
1963	0.41
1964	0.44
1965	0.4
1966	0.34
1967	0.41
1968	0.42
1969	0.38
1970	0.32
1971	0.34
1972	0.32
1973	0.32
1974	0.38
1975	0.4
1976	0.36
1977	0.33
1978	0.37
1979	0.45
1980	0.44
1981	0.47
1982	0.48
1983	0.48
1984	0.45
1985	0.47
1986	0.43
1987	0.39
1988	0.39
1989	0.41
1990	0.42
1991	0.39
1992	0.37
1993	0.35
1994	0.33
1995	0.31
1996	0.32
1997	0.28
1998	0.26
1999	0.26
2000	0.27
2001	0.27
2002	0.27
2003	0.28
2004	0.28
2005	0.36
2006	0.36
2007	0.37
2008	0.38