Year	Percent
1960	23.01865
1961	23.0714
1962	23.12007
1963	23.16542
1964	23.21
1965	23.25221
1966	23.29632
1967	23.34147
1968	23.38667
1969	23.42653
1970	23.43975
1971	23.3231
1972	23.2071
1973	23.09565
1974	22.99823
1975	22.91841
1976	22.85927
1977	22.81599
1978	22.78448
1979	22.75573
1980	22.72591
1981	22.69007
1982	22.65449
1983	22.62795
1984	22.62323
1985	22.67902
1986	22.83543
1987	23.01873
1988	23.21566
1989	23.40446
1990	23.57646
1991	23.79981
1992	24.17623
1993	24.56827
1994	25.00415
1995	25.50016
1996	26.06434
1997	26.55248
1998	27.03632
1999	27.55622
2000	28.10722
2001	28.68335
2002	29.06643
2003	29.38826
2004	29.73782
2005	30.11331
2006	30.52027
2007	30.95328
2008	31.39905
2009	31.83504
2010	32.25119
2011	32.64045
2012	33.01003
2013	33.37256
2014	33.74792
2015	34.15084
2016	34.58575
2017	35.0499