Year	Percent
1960	17.23819
1961	17.3784
1962	17.52085
1963	17.67344
1964	17.8491
1965	18.0448
1966	18.26395
1967	18.50511
1968	18.77262
1969	19.06622
1970	19.39075
1971	19.74657
1972	20.13222
1973	20.51186
1974	20.74179
1975	20.96684
1976	21.18604
1977	21.39818
1978	21.60656
1979	21.81158
1980	22.01556
1981	22.21452
1982	22.41223
1983	22.6127
1984	22.81936
1985	22.97976
1986	23.15527
1987	23.34317
1988	23.53919
1989	23.73291
1990	23.92339
1991	24.1105
1992	24.29935
1993	24.49024
1994	24.69094
1995	24.90463
1996	25.13212
1997	25.37252
1998	25.63951
1999	25.94463
2000	26.29084
2001	26.21749
2002	26.18505
2003	26.18273
2004	26.19766
2005	26.21871
2006	26.24477
2007	26.27779
2008	26.32026
2009	26.37408
2010	26.44321
2011	26.5272
2012	26.62468
2013	26.73272
2014	26.85131
2015	26.9786
2016	27.11478
2017	27.25846