Year	Percent
1960	44.198
1961	44.746
1962	45.296
1963	45.847
1964	46.4
1965	47.047
1966	47.696
1967	48.346
1968	48.997
1969	49.647
1970	50.236
1971	50.753
1972	51.269
1973	51.785
1974	52.3
1975	52.564
1976	52.828
1977	53.092
1978	53.356
1979	53.7
1980	54.142
1981	54.582
1982	55.022
1983	55.461
1984	55.9
1985	56.01
1986	56.121
1987	56.231
1988	56.341
1989	56.357
1990	56.266
1991	56.175
1992	56.083
1993	55.992
1994	55.9
1995	55.921
1996	55.943
1997	55.964
1998	55.986
1999	56.025
2000	56.098
2001	56.171
2002	56.244
2003	56.317
2004	56.39
2005	56.463
2006	56.536
2007	56.609
2008	56.682
2009	56.755
2010	56.827
2011	56.9
2012	56.973
2013	57.046
2014	57.118
2015	57.191
2016	57.264
2017	57.336