Year	Percent
1960	12.1916
1961	12.65048
1962	13.23175
1963	13.84409
1964	14.48993
1965	15.16635
1966	15.74763
1967	16.2975
1968	16.87711
1969	17.48271
1970	18.1193
1971	18.55175
1972	18.88815
1973	19.2333
1974	19.5938
1975	19.97093
1976	20.28318
1977	20.56893
1978	20.86777
1979	21.17447
1980	21.488
1981	21.86797
1982	22.28242
1983	22.71214
1984	23.16776
1985	23.65221
1986	24.10146
1987	24.55016
1988	25.02912
1989	25.53236
1990	26.06214
1991	26.44601
1992	26.77339
1993	27.11262
1994	27.46268
1995	27.81969
1996	28.1837
1997	28.55254
1998	28.87869
1999	29.17827
2000	29.49266
2001	29.95596
2002	30.49904
2003	31.0639
2004	31.65807
2005	32.2806
2006	32.94083
2007	33.63368
2008	34.34453
2009	35.0458
2010	35.72618
2011	36.38084
2012	36.74511
2013	36.81756
2014	36.8896
2015	36.93332
2016	36.9472
2017	36.97305