Year	Percent
1960	2.52869
1961	2.25425
1962	2.21417
1963	2.23391
1964	2.27152
1965	2.32885
1966	2.40214
1967	2.41768
1968	2.3439
1969	2.11526
1970	1.88212
1971	1.8032
1972	1.55719
1973	1.36036
1974	1.23019
1975	1.1614
1976	1.08891
1977	1.03619
1978	1.11393
1979	1.35055
1980	1.6675
1981	1.95064
1982	2.2426
1983	2.42249
1984	2.47045
1985	2.4184
1986	2.3833
1987	2.36724
1988	2.26697
1989	2.04987
1990	1.80222
1991	1.47743
1992	1.2069
1993	1.23356
1994	1.58174
1995	2.13327
1996	2.73587
1997	3.18623
1998	3.38414
1999	3.26418
2000	3.00435
2001	3.01615
2002	2.75564
2003	2.56872
2004	2.51558
2005	2.53256
2006	2.56572
2007	2.56298
2008	2.52852
2009	2.46687
2010	2.37929
2011	2.33071
2012	2.29535
2013	2.27657
2014	2.27608
2015	2.29288
2016	2.30739
2017	2.31624