Year	Percent
1960	2.34378
1961	2.35519
1962	2.3649
1963	2.37331
1964	2.38113
1965	2.38852
1966	2.39643
1967	2.40471
1968	2.46015
1969	2.51415
1970	2.56641
1971	2.61542
1972	2.66263
1973	2.71223
1974	2.77096
1975	2.84256
1976	2.92949
1977	3.02901
1978	3.13545
1979	3.23907
1980	3.33258
1981	3.41245
1982	3.48124
1983	3.54179
1984	3.59449
1985	3.61849
1986	3.64309
1987	3.66705
1988	3.68961
1989	3.70867
1990	3.72425
1991	3.73614
1992	3.74612
1993	3.75632
1994	3.77055
1995	3.74157
1996	3.70032
1997	3.66432
1998	3.63222
1999	3.60184
2000	3.57198
2001	3.54206
2002	3.51287
2003	3.48466
2004	3.45809
2005	3.43314
2006	3.41013
2007	3.39563
2008	3.45094
2009	3.50723
2010	3.56485
2011	3.62356
2012	3.68382
2013	3.74534
2014	3.80925
2015	3.87571
2016	3.94512
2017	4.01683