Year	Percent
1960	39.581
1961	40.03
1962	40.481
1963	41.005
1964	41.855
1965	42.707
1966	43.565
1967	44.427
1968	45.293
1969	46.16
1970	47.03
1971	47.785
1972	48.063
1973	48.341
1974	48.62
1975	48.898
1976	49.177
1977	49.456
1978	49.734
1979	50.013
1980	50.292
1981	50.57
1982	50.849
1983	51.127
1984	51.406
1985	51.684
1986	51.963
1987	52.241
1988	52.519
1989	52.797
1990	53.075
1991	53.352
1992	53.63
1993	53.907
1994	54.183
1995	54.437
1996	54.587
1997	54.737
1998	54.886
1999	55.036
2000	55.185
2001	55.335
2002	55.484
2003	55.633
2004	55.782
2005	55.935
2006	56.132
2007	56.328
2008	56.525
2009	56.721
2010	56.917
2011	57.113
2012	57.309
2013	57.505
2014	57.7
2015	57.895
2016	58.09
2017	58.299