Year	Percent
1960	30.154
1961	31.04
1962	32.001
1963	32.979
1964	33.972
1965	34.977
1966	35.997
1967	37.03
1968	38.076
1969	39.13
1970	40.196
1971	41.168
1972	41.839
1973	42.511
1974	43.186
1975	43.864
1976	44.545
1977	45.227
1978	45.911
1979	46.597
1980	47.285
1981	47.887
1982	48.237
1983	48.587
1984	48.937
1985	49.287
1986	49.637
1987	49.987
1988	50.338
1989	50.687
1990	51.037
1991	51.357
1992	51.588
1993	51.818
1994	52.048
1995	52.279
1996	52.509
1997	52.739
1998	52.968
1999	53.198
2000	53.428
2001	53.642
2002	53.81
2003	53.979
2004	54.147
2005	54.315
2006	54.483
2007	54.651
2008	54.819
2009	54.987
2010	55.155
2011	55.322
2012	55.506
2013	55.706
2014	55.923
2015	56.155
2016	56.403
2017	56.667