Year	Percent
1960	17.47
1961	17.37
1962	17.09
1963	17.7
1964	16.35
1965	16.96
1966	17.04
1967	15.88
1968	18.01
1969	18.13
1970	16.93
1971	16.62
1972	15.75
1973	15.36
1974	15.87
1975	16.84
1976	16.44
1977	15.87
1978	15.16
1979	15.16
1980	14.67
1981	15.08
1982	14.93
1983	14.93
1984	13.45
1985	13.33
1986	13.46
1987	13.08
1988	12.15
1989	12.31
1990	12.14
1991	12.02
1992	11.53
1993	11.39
1994	11.02
1995	10.67
1996	10.73
1997	10.73
1998	11.14
1999	10.65
2000	10.48
2001	10.71
2002	10.21
2003	11.11
2004	11.03
2005	11.43
2006	11.69
2007	11.35
2008	11.37
2009	12.16
2010	11.85