Year	Percent
1960	6.4281
1961	6.76069
1962	7.11215
1963	7.4817
1964	7.86953
1965	8.27312
1966	8.69522
1967	9.13719
1968	9.50639
1969	9.83164
1970	10.17455
1971	10.43394
1972	10.70886
1973	10.9937
1974	11.28591
1975	11.58172
1976	11.87979
1977	12.18002
1978	12.49228
1979	12.82562
1980	13.18753
1981	13.57677
1982	13.9925
1983	14.42767
1984	14.87441
1985	15.23043
1986	15.58616
1987	15.9406
1988	16.28109
1989	16.58718
1990	16.85402
1991	17.07249
1992	17.25749
1993	17.44309
1994	17.67914
1995	17.99285
1996	18.39561
1997	18.86909
1998	19.3189
1999	19.69119
2000	20.03154
2001	20.32913
2002	20.59656
2003	20.83916
2004	21.06944
2005	21.29436
2006	21.51462
2007	21.72815
2008	21.93682
2009	22.14249
2010	22.3469
2011	22.5516
2012	22.7581
2013	22.96804
2014	23.18305
2015	23.40452
2016	23.63334
2017	23.87042