Year	Percent
1960	10.472
1961	10.936
1962	11.418
1963	11.919
1964	12.439
1965	12.978
1966	13.537
1967	14.116
1968	14.716
1969	15.336
1970	15.977
1971	16.641
1972	17.327
1973	18.033
1974	18.763
1975	19.515
1976	20.291
1977	21.087
1978	21.907
1979	22.75
1980	23.617
1981	24.503
1982	25.414
1983	26.072
1984	26.346
1985	26.621
1986	26.898
1987	27.178
1988	27.459
1989	27.741
1990	28.026
1991	28.312
1992	28.601
1993	28.89
1994	29.182
1995	29.475
1996	29.77
1997	30.052
1998	30.323
1999	30.595
2000	30.869
2001	31.144
2002	31.42
2003	31.698
2004	31.977
2005	32.257
2006	32.539
2007	32.822
2008	33.106
2009	33.391
2010	33.678
2011	33.966
2012	34.255
2013	34.545
2014	34.836
2015	35.141
2016	35.46
2017	35.793