Year	Percent
1960	6.868
1961	7.051
1962	7.246
1963	7.445
1964	7.65
1965	7.86
1966	8.075
1967	8.296
1968	8.522
1969	8.753
1970	8.99
1971	9.298
1972	9.671
1973	10.056
1974	10.456
1975	10.869
1976	11.297
1977	11.739
1978	12.197
1979	12.669
1980	13.158
1981	14.051
1982	15.035
1983	16.074
1984	17.172
1985	18.326
1986	19.541
1987	20.815
1988	22.152
1989	23.545
1990	25
1991	25.487
1992	25.98
1993	26.479
1994	26.984
1995	27.495
1996	28.013
1997	28.535
1998	28.742
1999	28.92
2000	29.098
2001	29.277
2002	29.457
2003	29.637
2004	29.818
2005	29.999
2006	30.182
2007	30.364
2008	30.832
2009	31.328
2010	31.83
2011	32.336
2012	32.846
2013	33.36
2014	33.878
2015	34.4
2016	34.926
2017	35.455