Year	Percent
1960	43.069
1961	43.559
1962	44.051
1963	44.544
1964	45.039
1965	45.534
1966	46.03
1967	46.527
1968	47.025
1969	47.522
1970	48.006
1971	48.474
1972	48.942
1973	49.409
1974	49.876
1975	50.344
1976	50.812
1977	51.279
1978	51.747
1979	52.156
1980	52.497
1981	52.836
1982	53.176
1983	53.516
1984	53.855
1985	54.194
1986	54.532
1987	54.87
1988	55.208
1989	55.277
1990	55.038
1991	54.799
1992	54.559
1993	54.32
1994	54.08
1995	53.841
1996	53.6
1997	53.36
1998	53.12
1999	52.879
2000	52.638
2001	52.398
2002	52.441
2003	52.83
2004	53.218
2005	53.605
2006	53.992
2007	54.379
2008	54.765
2009	55.15
2010	55.535
2011	55.919
2012	56.303
2013	56.685
2014	57.067
2015	57.448
2016	57.837
2017	58.231