Year	Percent
1960	52.663
1961	52.364
1962	52.064
1963	51.764
1964	51.464
1965	51.164
1966	50.864
1967	50.564
1968	50.263
1969	49.963
1970	50.008
1971	50.462
1972	50.917
1973	51.371
1974	51.698
1975	51.897
1976	52.096
1977	52.294
1978	52.492
1979	52.655
1980	52.777
1981	52.897
1982	53.018
1983	53.139
1984	53.299
1985	53.498
1986	53.697
1987	53.895
1988	54.094
1989	54.057
1990	53.749
1991	53.441
1992	53.132
1993	52.824
1994	52.515
1995	52.206
1996	51.896
1997	51.588
1998	51.278
1999	51.185
2000	51.386
2001	51.587
2002	51.787
2003	51.988
2004	52.189
2005	52.389
2006	52.589
2007	52.79
2008	52.99
2009	53.19
2010	53.406
2011	53.637
2012	53.883
2013	54.145
2014	54.422
2015	54.714
2016	55.021
2017	55.343