Year	Percent
1960	3.88836
1961	4.05237
1962	4.28521
1963	4.53211
1964	4.79663
1965	5.07968
1966	5.38467
1967	5.7107
1968	6.05542
1969	6.41169
1970	6.77685
1971	7.05314
1972	7.24541
1973	7.43012
1974	7.61139
1975	7.79051
1976	7.96978
1977	8.14676
1978	8.31984
1979	8.48398
1980	8.63782
1981	8.77743
1982	8.90879
1983	9.04313
1984	9.38058
1985	9.75263
1986	10.1687
1987	10.62513
1988	11.11241
1989	11.61017
1990	12.10907
1991	12.60146
1992	13.094
1993	13.59626
1994	14.01984
1995	14.48109
1996	14.98695
1997	15.52687
1998	16.09036
1999	16.65478
2000	17.20597
2001	17.73459
2002	18.25071
2003	18.7603
2004	19.27661
2005	19.80331
2006	20.34814
2007	20.9083
2008	21.48597
2009	22.07505
2010	22.68149
2011	23.30453
2012	23.94935
2013	24.61458
2014	25.22006
2015	25.20955
2016	25.21547
2017	25.23266