Year	Percent
1960	3.13517
1961	3.49359
1962	3.8938
1963	4.33876
1964	4.83236
1965	5.37618
1966	5.97652
1967	6.64019
1968	7.38299
1969	8.2223
1970	9.18496
1971	10.29025
1972	11.55811
1973	12.31796
1974	12.55937
1975	12.81231
1976	13.07666
1977	13.34639
1978	13.60456
1979	13.82673
1980	13.99512
1981	14.09552
1982	14.13174
1983	14.18109
1984	14.29386
1985	14.39104
1986	14.4914
1987	14.58564
1988	14.63543
1989	14.59038
1990	14.4347
1991	14.16839
1992	13.84
1993	13.52415
1994	13.29942
1995	13.20211
1996	13.23454
1997	13.28751
1998	13.34883
1999	13.43853
2000	13.52675
2001	13.60697
2002	13.68721
2003	13.76318
2004	13.83328
2005	13.89472
2006	13.94504
2007	13.98251
2008	14.01181
2009	14.03773
2010	14.06548
2011	14.09584
2012	14.12684
2013	14.15284
2014	14.1698
2015	14.17403
2016	14.16518
2017	14.1461