Year	Percent
1960	11.066
1961	11.361
1962	11.662
1963	11.97
1964	12.286
1965	12.608
1966	12.938
1967	13.274
1968	13.619
1969	13.971
1970	14.33
1971	14.697
1972	15.072
1973	15.454
1974	15.845
1975	16.244
1976	16.651
1977	17.083
1978	17.54
1979	18.007
1980	18.484
1981	18.97
1982	19.466
1983	19.972
1984	20.488
1985	21.013
1986	21.548
1987	22.069
1988	22.482
1989	22.899
1990	23.322
1991	23.751
1992	24.186
1993	24.624
1994	25.069
1995	25.519
1996	25.975
1997	26.434
1998	26.948
1999	27.646
2000	28.356
2001	29.075
2002	29.806
2003	30.547
2004	31.3
2005	32.06
2006	32.832
2007	33.612
2008	34.403
2009	35.2
2010	35.999
2011	36.799
2012	37.599
2013	38.398
2014	39.196
2015	39.991
2016	40.783
2017	41.572