Year	Percent
1960	11.55
1961	10.99
1962	10.91
1963	10.18
1964	9.199
1965	9.494
1966	8.512
1967	7.587
1968	6.795
1969	6.378
1970	9.456
1971	8.624
1972	8.587
1973	8.943
1974	9.229
1975	10
1976	10.42
1977	10.56
1978	10.8
1979	10.72
1980	11.1
1981	11.98
1982	13.17
1983	13.81
1984	14.13
1985	14.24
1986	14.42
1987	14.43
1988	14.61
1989	14.08
1990	12.66
1991	12.76
1992	13.19
1993	13.75
1994	13.4
1995	13.66
1996	13.51
1997	13.4
1998	13.76
1999	13.76
2000	14.09
2001	13.9
2002	14.19
2003	13.88
2004	13.9
2005	14.46
2006	14
2007	14.36
2008	14.83
2009	14.93
2010	15.31