Year	Percent
1960	37.1
1961	38.782
1962	40.494
1963	42.23
1964	43.987
1965	45.755
1966	47.233
1967	48.499
1968	49.768
1969	51.034
1970	52.3
1971	53.36
1972	54.419
1973	55.471
1974	56.519
1975	57.562
1976	58.526
1977	59.428
1978	60.326
1979	61.216
1980	62.1
1981	62.603
1982	63.104
1983	63.603
1984	64.099
1985	64.59
1986	64.999
1987	65.346
1988	65.692
1989	66.035
1990	66.377
1991	66.717
1992	67.056
1993	67.33
1994	67.557
1995	67.782
1996	68.007
1997	68.231
1998	68.454
1999	68.677
2000	68.899
2001	69.166
2002	69.524
2003	69.879
2004	70.233
2005	70.584
2006	70.932
2007	71.278
2008	71.622
2009	71.963
2010	72.302
2011	72.638
2012	72.975
2013	73.313
2014	73.651
2015	73.99
2016	74.329
2017	74.669