Year	Percent
1960	18.145
1961	18.951
1962	19.785
1963	20.712
1964	22.015
1965	23.372
1966	24.789
1967	26.261
1968	27.791
1969	29.37
1970	30.351
1971	31.241
1972	32.146
1973	33.061
1974	33.992
1975	34.934
1976	35.89
1977	36.855
1978	37.832
1979	38.818
1980	39.815
1981	39.854
1982	39.805
1983	39.755
1984	39.705
1985	39.655
1986	39.606
1987	39.556
1988	39.506
1989	39.456
1990	39.407
1991	38.989
1992	38.514
1993	38.042
1994	37.572
1995	37.104
1996	36.638
1997	36.176
1998	35.716
1999	35.258
2000	34.802
2001	35.002
2002	35.475
2003	35.951
2004	36.43
2005	36.911
2006	37.395
2007	37.881
2008	38.371
2009	38.861
2010	39.355
2011	39.851
2012	40.354
2013	40.865
2014	41.382
2015	41.907
2016	42.438
2017	42.976