Year	Percent
1960	54.028
1961	53.72
1962	53.41
1963	53.101
1964	52.79
1965	52.481
1966	52.17
1967	51.86
1968	51.549
1969	51.238
1970	50.963
1971	50.805
1972	50.646
1973	50.488
1974	50.329
1975	50.171
1976	50.012
1977	49.854
1978	49.695
1979	49.537
1980	49.374
1981	49.183
1982	48.992
1983	48.801
1984	48.61
1985	48.419
1986	48.228
1987	48.037
1988	47.846
1989	47.655
1990	47.465
1991	47.271
1992	47.059
1993	46.848
1994	46.637
1995	46.425
1996	46.214
1997	46.003
1998	45.793
1999	45.582
2000	45.398
2001	45.381
2002	45.364
2003	45.347
2004	45.33
2005	45.313
2006	45.296
2007	45.279
2008	45.262
2009	45.245
2010	45.228
2011	45.228
2012	45.246
2013	45.282
2014	45.335
2015	45.406
2016	45.495
2017	45.601