Year	Percent
1960	52.867
1961	53.663
1962	54.459
1963	55.253
1964	56.045
1965	56.832
1966	57.617
1967	58.398
1968	59.175
1969	59.946
1970	60.713
1971	61.433
1972	62.122
1973	62.805
1974	63.484
1975	64.157
1976	64.826
1977	65.487
1978	66.143
1979	66.686
1980	67.095
1981	67.44
1982	67.722
1983	68.003
1984	68.283
1985	68.56
1986	68.782
1987	68.947
1988	69.112
1989	69.2
1990	69.25
1991	69.201
1992	68.852
1993	68.799
1994	68.901
1995	68.75
1996	68.65
1997	68.6
1998	68.451
1999	68.22
2000	68.067
2001	67.95
2002	67.85
2003	67.8
2004	67.9
2005	68
2006	67.967
2007	67.9
2008	67.834
2009	67.814
2010	67.841
2011	67.869
2012	67.897
2013	67.925
2014	67.952
2015	67.98
2016	68.021
2017	68.075