Year	Percent
1960	33.769
1961	34.5
1962	35.239
1963	35.985
1964	36.74
1965	37.499
1966	38.265
1967	39.036
1968	39.815
1969	40.596
1970	41.324
1971	41.858
1972	42.395
1973	42.933
1974	43.472
1975	44.013
1976	44.557
1977	45.1
1978	45.645
1979	46.191
1980	46.739
1981	47.286
1982	47.813
1983	48.016
1984	48.22
1985	48.424
1986	48.628
1987	48.832
1988	49.036
1989	49.24
1990	49.444
1991	49.656
1992	49.896
1993	50.136
1994	50.375
1995	50.615
1996	50.855
1997	51.095
1998	51.335
1999	51.574
2000	51.814
2001	52.053
2002	52.251
2003	52.437
2004	52.625
2005	52.811
2006	52.998
2007	53.184
2008	53.371
2009	53.557
2010	53.743
2011	53.93
2012	54.132
2013	54.35
2014	54.584
2015	54.833
2016	55.098
2017	55.378