Year	Percent
1960	28.204
1961	29.107
1962	29.934
1963	30.776
1964	31.631
1965	32.497
1966	33.376
1967	34.266
1968	35.169
1969	36.081
1970	37.004
1971	37.972
1972	39.058
1973	40.153
1974	41.259
1975	42.374
1976	43.499
1977	44.627
1978	45.762
1979	46.901
1980	48.045
1981	48.941
1982	49.102
1983	49.263
1984	49.424
1985	49.584
1986	49.746
1987	49.907
1988	50.068
1989	50.229
1990	50.39
1991	50.517
1992	50.543
1993	50.569
1994	50.596
1995	50.622
1996	50.649
1997	50.675
1998	50.701
1999	50.728
2000	50.754
2001	50.78
2002	50.857
2003	51.082
2004	51.308
2005	51.533
2006	51.758
2007	51.983
2008	52.209
2009	52.433
2010	52.658
2011	52.883
2012	53.108
2013	53.332
2014	53.557
2015	53.781
2016	54.02
2017	54.273