Year	Percent
1960	29.681
1961	30.248
1962	30.823
1963	31.403
1964	31.991
1965	32.582
1966	33.18
1967	33.604
1968	33.987
1969	34.371
1970	34.758
1971	35.147
1972	35.538
1973	35.93
1974	36.324
1975	36.721
1976	37.119
1977	37.319
1978	37.469
1979	37.619
1980	37.769
1981	37.919
1982	38.07
1983	38.221
1984	38.372
1985	38.523
1986	38.675
1987	39.327
1988	40.085
1989	40.845
1990	41.611
1991	42.381
1992	43.156
1993	43.933
1994	44.713
1995	45.496
1996	46.282
1997	46.732
1998	47.123
1999	47.515
2000	47.908
2001	48.3
2002	48.692
2003	49.085
2004	49.479
2005	49.871
2006	50.264
2007	50.657
2008	51.146
2009	51.658
2010	52.171
2011	52.683
2012	53.196
2013	53.706
2014	54.216
2015	54.726
2016	55.235
2017	55.742