Year	Percent
1960	51.275
1961	52.147
1962	53.019
1963	53.889
1964	54.758
1965	55.622
1966	56.483
1967	57.341
1968	58.195
1969	59.042
1970	59.864
1971	60.657
1972	61.445
1973	62.225
1974	63
1975	63.611
1976	64.219
1977	64.821
1978	65.419
1979	65.839
1980	66.05
1981	66.259
1982	66.467
1983	66.675
1984	66.883
1985	67.09
1986	67.296
1987	67.501
1988	67.706
1989	67.69
1990	67.421
1991	67.15
1992	66.878
1993	66.606
1994	66.332
1995	66.057
1996	65.781
1997	65.504
1998	65.226
1999	64.947
2000	64.666
2001	64.385
2002	64.236
2003	64.137
2004	64.038
2005	63.938
2006	63.839
2007	63.739
2008	63.64
2009	63.54
2010	63.44
2011	63.34
2012	63.24
2013	63.164
2014	63.112
2015	63.085
2016	63.082
2017	63.103