Year	Percent
1960	37.511
1961	37.95
1962	38.391
1963	38.833
1964	39.279
1965	39.724
1966	40.23
1967	41.036
1968	41.848
1969	42.662
1970	43.482
1971	44.305
1972	45.132
1973	45.959
1974	46.79
1975	47.588
1976	48.184
1977	48.779
1978	49.375
1979	49.972
1980	50.569
1981	51.164
1982	51.76
1983	52.356
1984	53.011
1985	53.839
1986	54.667
1987	55.491
1988	56.314
1989	57.131
1990	57.946
1991	58.756
1992	59.562
1993	60.362
1994	61.078
1995	61.474
1996	61.87
1997	62.262
1998	62.653
1999	63.043
2000	63.432
2001	63.818
2002	64.202
2003	64.585
2004	64.95
2005	65.237
2006	65.524
2007	65.809
2008	66.093
2009	66.376
2010	66.657
2011	66.938
2012	67.218
2013	67.495
2014	67.772
2015	68.056
2016	68.346
2017	68.642