Year	Percent
1960	34.02
1961	35.214
1962	36.473
1963	37.75
1964	39.047
1965	40.355
1966	41.68
1967	43.016
1968	44.364
1969	45.718
1970	47.079
1971	48.246
1972	48.827
1973	49.407
1974	49.988
1975	50.569
1976	51.151
1977	51.73
1978	52.31
1979	52.889
1980	53.469
1981	54.007
1982	54.43
1983	54.853
1984	55.276
1985	55.696
1986	56.117
1987	56.536
1988	56.955
1989	57.373
1990	57.789
1991	58.234
1992	58.764
1993	59.291
1994	59.794
1995	59.587
1996	59.38
1997	59.172
1998	58.965
1999	58.757
2000	58.548
2001	58.339
2002	58.13
2003	57.921
2004	57.711
2005	57.532
2006	57.383
2007	57.264
2008	57.175
2009	57.117
2010	57.089
2011	57.092
2012	57.125
2013	57.189
2014	57.284
2015	57.408
2016	57.563
2017	57.748