Year	Percent
1960	11.173
1961	11.23
1962	11.288
1963	11.345
1964	11.404
1965	11.458
1966	11.393
1967	11.336
1968	11.516
1969	11.698
1970	11.893
1971	12.842
1972	13.858
1973	14.936
1974	16.085
1975	17.304
1976	18.596
1977	19.958
1978	20.978
1979	21.607
1980	22.25
1981	22.905
1982	23.574
1983	24.257
1984	24.954
1985	25.491
1986	25.57
1987	25.648
1988	25.727
1989	25.806
1990	25.84
1991	25.776
1992	25.713
1993	25.649
1994	25.586
1995	25.638
1996	26.008
1997	26.379
1998	26.755
1999	27.134
2000	27.706
2001	28.859
2002	30.042
2003	31.252
2004	32.49
2005	33.75
2006	34.794
2007	35.2
2008	35.61
2009	36.021
2010	36.434
2011	36.849
2012	37.267
2013	37.685
2014	38.106
2015	38.529
2016	38.954
2017	39.38