Year	Percent
1960	55.123
1961	55.345
1962	55.586
1963	55.826
1964	56.066
1965	56.305
1966	56.47
1967	56.311
1968	56.151
1969	55.991
1970	55.831
1971	55.556
1972	54.77
1973	53.983
1974	53.194
1975	52.403
1976	51.799
1977	51.795
1978	51.791
1979	51.788
1980	51.784
1981	51.78
1982	51.776
1983	51.773
1984	51.769
1985	51.765
1986	51.761
1987	51.757
1988	51.754
1989	51.75
1990	51.746
1991	51.745
1992	51.754
1993	51.764
1994	51.773
1995	51.783
1996	51.792
1997	51.8
1998	51.808
1999	51.815
2000	51.823
2001	51.833
2002	51.851
2003	51.869
2004	51.887
2005	51.905
2006	51.922
2007	51.94
2008	51.958
2009	51.976
2010	51.994
2011	52.012
2012	52.046
2013	52.096
2014	52.162
2015	52.245
2016	52.343
2017	52.458