Year	Percent
1960	15.71
1961	14.97
1962	15.77
1963	16.19
1964	16.01
1965	15.74
1966	15.88
1967	15.3
1968	15.94
1969	15.78
1970	16.15
1971	15.28
1972	14.98
1973	14.03
1974	13.49
1975	14.29
1976	19.53
1977	20.08
1978	19.68
1979	19.7
1980	19.94
1981	20.05
1982	20.71
1983	20.12
1984	19.95
1985	20.21
1986	21.03
1987	20.77
1988	20.35
1989	20.05
1990	20.67
1991	21.28
1992	21.23
1993	20.96
1994	21.35
1995	20.79
1996	20.94
1997	20.34
1998	20.19
1999	19.85
2000	19.32
2001	19.55
2002	19.92
2003	18.86
2004	18.26
2005	18.96
2006	18.71
2007	18.4
2008	18.31
2009	19.96
2010	20.21