Year	Percent
1960	27.673
1961	28.717
1962	29.786
1963	30.877
1964	31.991
1965	33.123
1966	34.277
1967	35.449
1968	36.641
1969	37.846
1970	39.068
1971	40.337
1972	41.802
1973	43.277
1974	44.767
1975	46.266
1976	47.774
1977	49.282
1978	49.396
1979	49.381
1980	49.366
1981	49.352
1982	49.337
1983	49.322
1984	49.307
1985	49.292
1986	49.277
1987	49.262
1988	49.247
1989	49.25
1990	49.27
1991	49.307
1992	49.362
1993	49.435
1994	49.525
1995	49.633
1996	49.758
1997	49.901
1998	50.061
1999	50.238
2000	50.433
2001	50.646
2002	50.876
2003	51.123
2004	51.388
2005	51.67
2006	51.969
2007	52.285
2008	52.619
2009	52.969
2010	53.336
2011	53.72
2012	54.12
2013	54.537
2014	54.969
2015	55.4
2016	55.831
2017	56.261