Year	Percent
1960	6.40157
1961	6.67636
1962	6.96267
1963	7.26074
1964	7.57197
1965	7.89533
1966	8.23294
1967	8.58507
1968	8.95525
1969	9.34484
1970	9.75895
1971	10.19892
1972	10.46859
1973	10.70588
1974	10.94535
1975	11.18236
1976	11.41688
1977	11.64758
1978	11.87504
1979	12.09746
1980	12.31521
1981	12.52564
1982	12.73226
1983	13.05838
1984	13.42048
1985	13.79751
1986	14.19447
1987	14.6106
1988	15.04743
1989	15.5016
1990	15.97625
1991	16.47164
1992	16.99042
1993	17.39522
1994	17.81612
1995	18.25204
1996	18.70414
1997	18.85034
1998	19.00425
1999	19.16686
2000	19.8085
2001	20.47806
2002	21.18033
2003	21.91322
2004	22.66159
2005	23.43227
2006	24.23197
2007	25.06139
2008	25.92523
2009	26.82227
2010	21.4072
2011	21.65781
2012	21.91938
2013	22.19025
2014	22.47265
2015	22.76713
2016	23.07426
2017	23.39429