Year	Percent
1960	23.252
1961	23.797
1962	24.352
1963	24.915
1964	25.488
1965	26.067
1966	26.656
1967	27.254
1968	27.86
1969	28.473
1970	28.958
1971	29.174
1972	29.392
1973	29.61
1974	29.829
1975	30.049
1976	30.27
1977	30.492
1978	30.714
1979	30.938
1980	31.163
1981	31.388
1982	31.614
1983	31.842
1984	32.209
1985	32.896
1986	33.591
1987	34.293
1988	35.004
1989	35.719
1990	36.441
1991	37.17
1992	37.905
1993	38.644
1994	39.39
1995	40.14
1996	40.896
1997	41.654
1998	42.418
1999	43.184
2000	43.929
2001	44.601
2002	45.275
2003	45.951
2004	46.63
2005	47.308
2006	47.988
2007	48.669
2008	49.351
2009	50.031
2010	50.713
2011	51.394
2012	52.073
2013	52.748
2014	53.419
2015	54.086
2016	54.749
2017	55.407