Year	Percent
1960	4.02274
1961	3.51724
1962	3.54617
1963	3.58151
1964	3.63697
1965	3.70554
1966	3.77827
1967	3.82941
1968	3.84793
1969	3.81758
1970	3.76429
1971	3.69233
1972	3.64328
1973	3.61886
1974	3.6501
1975	3.70802
1976	3.77932
1977	3.83112
1978	3.81995
1979	3.73872
1980	3.6134
1981	3.49214
1982	3.4085
1983	3.37769
1984	3.42108
1985	4.04194
1986	4.10151
1987	4.15465
1988	4.2963
1989	4.5232
1990	4.79315
1991	5.09434
1992	5.31114
1993	5.29249
1994	5.02157
1995	4.60358
1996	4.14924
1997	3.79886
1998	3.62436
1999	3.66847
2000	3.86058
2001	4.07639
2002	4.24151
2003	4.36903
2004	4.43867
2005	4.47833
2006	4.51859
2007	4.56709
2008	4.60028
2009	4.62912
2010	4.6441
2011	4.65664
2012	4.6571
2013	4.65026
2014	4.63889
2015	4.61933
2016	4.59644
2017	4.56797