Year	Percent
1960	3.03776
1961	3.16694
1962	3.2788
1963	3.35802
1964	3.39767
1965	3.40379
1966	3.41447
1967	3.41279
1968	3.34912
1969	3.21515
1970	3.04427
1971	2.86285
1972	2.71958
1973	2.64823
1974	2.66764
1975	2.74754
1976	2.84707
1977	2.92833
1978	2.98393
1979	3.00259
1980	3.00083
1981	2.98823
1982	2.99504
1983	3.04503
1984	3.14754
1985	3.27674
1986	3.4089
1987	3.50695
1988	3.54955
1989	3.52547
1990	3.45852
1991	3.3912
1992	3.33852
1993	3.28298
1994	3.22725
1995	3.17718
1996	3.1226
1997	3.08287
1998	3.08846
1999	3.14793
2000	3.23952
2001	3.33657
2002	3.41384
2003	3.46509
2004	3.48276
2005	3.47732
2006	3.46615
2007	3.45829
2008	3.4488
2009	3.43942
2010	3.429
2011	3.41588
2012	3.39848
2013	3.37677
2014	3.35065
2015	3.32156
2016	3.29087
2017	3.26046