Year	Percent
1960	30.332
1961	30.519
1962	30.707
1963	30.895
1964	31.085
1965	31.274
1966	31.465
1967	31.656
1968	31.848
1969	32.04
1970	32.212
1971	32.281
1972	32.351
1973	32.421
1974	32.491
1975	32.561
1976	32.631
1977	32.701
1978	32.771
1979	32.841
1980	32.912
1981	32.978
1982	33.028
1983	33.078
1984	33.128
1985	33.177
1986	33.227
1987	33.277
1988	33.327
1989	33.377
1990	33.427
1991	33.503
1992	33.744
1993	33.985
1994	34.227
1995	34.47
1996	34.714
1997	34.959
1998	35.204
1999	35.45
2000	35.698
2001	35.919
2002	35.913
2003	35.907
2004	35.901
2005	35.895
2006	35.889
2007	35.883
2008	35.877
2009	35.871
2010	35.865
2011	35.859
2012	35.869
2013	35.896
2014	35.938
2015	35.997
2016	36.072
2017	36.164