Year	Percent
1960	26.57702
1961	26.75983
1962	26.97363
1963	27.27625
1964	27.67956
1965	28.21859
1966	28.86907
1967	29.49974
1968	30.08313
1969	30.82178
1970	31.57631
1971	32.39232
1972	33.30873
1973	34.18635
1974	34.6142
1975	34.21551
1976	34.14878
1977	34.69369
1978	35.24368
1979	35.80605
1980	36.37647
1981	36.77143
1982	36.63434
1983	36.55164
1984	36.49828
1985	36.48532
1986	36.53918
1987	36.63637
1988	36.76228
1989	36.90394
1990	37.07302
1991	37.27756
1992	37.5391
1993	37.72633
1994	37.85923
1995	37.96336
1996	38.05681
1997	38.12299
1998	38.16716
1999	38.18841
2000	38.15764
2001	38.10083
2002	38.04834
2003	38.06151
2004	38.12689
2005	38.21249
2006	38.30058
2007	38.3828
2008	38.48555
2009	38.60707
2010	38.74841
2011	38.96546
2012	39.28517
2013	39.6644
2014	40.04352
2015	40.3758
2016	40.67077
2017	40.96485