Year	Percent
1960	30.297
1961	30.555
1962	30.816
1963	31.077
1964	31.34
1965	31.604
1966	31.869
1967	32.135
1968	32.403
1969	32.672
1970	32.979
1971	33.493
1972	34.012
1973	34.533
1974	35.059
1975	35.56
1976	35.917
1977	36.275
1978	36.635
1979	36.997
1980	37.456
1981	38.394
1982	39.343
1983	40.3
1984	41.266
1985	42.235
1986	43.212
1987	44.195
1988	45.183
1989	46.172
1990	46.986
1991	46.901
1992	46.815
1993	46.73
1994	46.645
1995	46.56
1996	46.475
1997	46.39
1998	46.305
1999	46.22
2000	46.135
2001	46.05
2002	45.965
2003	45.88
2004	45.795
2005	45.71
2006	45.625
2007	45.54
2008	45.455
2009	45.371
2010	45.332
2011	45.522
2012	45.712
2013	45.903
2014	46.093
2015	46.284
2016	46.475
2017	46.682