Year	Percent
1960	16.431
1961	16.592
1962	16.753
1963	16.916
1964	17.014
1965	17.11
1966	17.207
1967	17.304
1968	17.402
1969	17.5
1970	17.598
1971	17.697
1972	17.796
1973	17.896
1974	17.996
1975	18.097
1976	18.198
1977	18.299
1978	18.401
1979	18.504
1980	18.607
1981	18.676
1982	18.66
1983	18.644
1984	18.629
1985	18.613
1986	18.597
1987	18.582
1988	18.566
1989	18.551
1990	18.535
1991	18.52
1992	18.504
1993	18.489
1994	18.473
1995	18.458
1996	18.442
1997	18.427
1998	18.411
1999	18.396
2000	18.38
2001	18.365
2002	18.349
2003	18.334
2004	18.319
2005	18.303
2006	18.288
2007	18.272
2008	18.257
2009	18.242
2010	18.226
2011	18.211
2012	18.196
2013	18.198
2014	18.218
2015	18.256
2016	18.311
2017	18.384