Year	Percent
1960	34.155
1961	35.004
1962	35.864
1963	36.734
1964	37.613
1965	38.498
1966	39.392
1967	40.294
1968	41.203
1969	42.115
1970	43.035
1971	43.959
1972	44.888
1973	45.819
1974	46.754
1975	47.691
1976	48.631
1977	49.57
1978	50.51
1979	51.449
1980	52.284
1981	52.468
1982	52.653
1983	52.837
1984	53.021
1985	53.205
1986	53.389
1987	53.573
1988	53.757
1989	53.941
1990	54.124
1991	54.308
1992	54.491
1993	54.674
1994	54.857
1995	55.04
1996	55.224
1997	55.406
1998	55.589
1999	55.771
2000	55.905
2001	55.718
2002	55.53
2003	55.343
2004	55.155
2005	54.967
2006	54.779
2007	54.59
2008	54.402
2009	54.213
2010	54.025
2011	53.836
2012	53.671
2013	53.53
2014	53.413
2015	53.319
2016	53.25
2017	53.205