Year	Percent
1960	7.362
1961	7.565
1962	7.774
1963	8.038
1964	8.318
1965	8.605
1966	8.902
1967	9.209
1968	9.525
1969	9.85
1970	10.295
1971	10.778
1972	11.282
1973	11.805
1974	12.349
1975	12.914
1976	13.503
1977	14.112
1978	14.745
1979	15.401
1980	15.583
1981	15.681
1982	15.78
1983	15.879
1984	15.979
1985	16.079
1986	16.18
1987	16.281
1988	16.383
1989	16.485
1990	16.748
1991	17.043
1992	17.342
1993	17.645
1994	17.952
1995	18.263
1996	18.579
1997	18.898
1998	19.222
1999	19.55
2000	19.892
2001	20.239
2002	20.591
2003	20.948
2004	21.31
2005	21.675
2006	22.045
2007	22.42
2008	22.8
2009	23.183
2010	23.571
2011	23.969
2012	24.376
2013	24.794
2014	25.221
2015	25.658
2016	26.105
2017	26.562