Year	Percent
1960	17.352
1961	17.903
1962	18.467
1963	19.06
1964	19.712
1965	20.379
1966	21.064
1967	21.765
1968	22.484
1969	23.218
1970	23.969
1971	24.737
1972	25.522
1973	26.321
1974	27.137
1975	27.729
1976	28.14
1977	28.554
1978	28.972
1979	29.394
1980	29.819
1981	30.248
1982	30.68
1983	31.115
1984	31.555
1985	31.997
1986	32.325
1987	32.555
1988	32.787
1989	33.019
1990	33.252
1991	33.485
1992	33.72
1993	33.955
1994	34.192
1995	34.429
1996	34.667
1997	34.905
1998	35.144
1999	35.384
2000	35.626
2001	35.867
2002	36.109
2003	36.352
2004	36.596
2005	36.919
2006	37.303
2007	37.689
2008	38.077
2009	38.466
2010	38.856
2011	39.248
2012	39.642
2013	40.036
2014	40.432
2015	40.829
2016	41.228
2017	41.636