Year	Percent
1960	61.52385
1961	61.52377
1962	60.44527
1963	59.29392
1964	58.97321
1965	59.70724
1966	61.59545
1967	64.02929
1968	66.64847
1969	69.26406
1970	69.34637
1971	67.77783
1972	65.74258
1973	63.75719
1974	62.21476
1975	61.28419
1976	59.67342
1977	57.5283
1978	56.36154
1979	56.36896
1980	57.58511
1981	57.90597
1982	57.3394
1983	57.2141
1984	57.17046
1985	56.97803
1986	56.6948
1987	56.38962
1988	56.05614
1989	55.75006
1990	55.52303
1991	55.31973
1992	55.12377
1993	54.99843
1994	55.03958
1995	55.26601
1996	55.91928
1997	56.84442
1998	57.74092
1999	58.56242
2000	59.24036
2001	60.01016
2002	60.78691
2003	60.86131
2004	59.52167
2005	56.7618
2006	53.09908
2007	49.39858
2008	46.46204
2009	44.8228
2010	44.57971
2011	44.95722
2012	46.32942
2013	48.41874
2014	50.84656
2015	53.29267
2016	55.67288
2017	58.07824