Year	Percent
1960	35.569
1961	35.67
1962	35.771
1963	35.916
1964	36.079
1965	36.243
1966	36.407
1967	36.571
1968	36.735
1969	36.9
1970	37.065
1971	37.23
1972	37.396
1973	37.916
1974	38.447
1975	38.981
1976	39.518
1977	40.056
1978	40.598
1979	41.141
1980	41.688
1981	42.235
1982	42.785
1983	43.512
1984	44.247
1985	44.983
1986	45.722
1987	46.463
1988	47.207
1989	47.949
1990	48.694
1991	49.439
1992	50.186
1993	50.844
1994	51.488
1995	52.131
1996	52.774
1997	53.414
1998	54.054
1999	54.693
2000	55.331
2001	55.966
2002	56.6
2003	56.977
2004	57.305
2005	57.633
2006	57.96
2007	58.286
2008	58.612
2009	58.937
2010	59.261
2011	59.584
2012	59.907
2013	60.197
2014	60.474
2015	60.75
2016	61.026
2017	61.3