Year	Percent
1960	31.121
1961	31.768
1962	32.424
1963	33.086
1964	33.682
1965	33.989
1966	34.298
1967	34.608
1968	34.919
1969	35.232
1970	35.545
1971	35.861
1972	36.177
1973	36.494
1974	36.809
1975	37.125
1976	37.443
1977	37.762
1978	38.081
1979	38.402
1980	38.724
1981	39.047
1982	39.37
1983	39.695
1984	40.021
1985	40.347
1986	40.674
1987	41.002
1988	41.332
1989	41.661
1990	41.992
1991	42.323
1992	42.655
1993	42.987
1994	43.32
1995	43.654
1996	43.989
1997	44.323
1998	44.659
1999	44.995
2000	45.332
2001	45.668
2002	46.005
2003	46.318
2004	46.616
2005	46.913
2006	47.21
2007	47.508
2008	47.807
2009	48.104
2010	48.403
2011	48.701
2012	49
2013	49.311
2014	49.634
2015	49.971
2016	50.319
2017	50.68