Year	Percent
1960	23.424
1961	24.216
1962	25.027
1963	25.856
1964	26.703
1965	27.566
1966	28.447
1967	29.345
1968	30.26
1969	31.189
1970	32.068
1971	32.879
1972	33.701
1973	34.531
1974	35.372
1975	36.222
1976	37.082
1977	37.948
1978	38.822
1979	39.64
1980	40.387
1981	41.136
1982	41.891
1983	42.649
1984	43.412
1985	44.176
1986	44.944
1987	45.714
1988	46.488
1989	46.856
1990	46.762
1991	46.669
1992	46.575
1993	46.481
1994	46.387
1995	46.294
1996	46.2
1997	45.797
1998	45.394
1999	44.992
2000	44.589
2001	44.189
2002	43.789
2003	43.389
2004	42.99
2005	42.775
2006	42.744
2007	42.713
2008	42.682
2009	42.651
2010	42.62
2011	42.589
2012	42.558
2013	42.527
2014	42.496
2015	42.49
2016	42.511
2017	42.557