Year	Percent
1960	5.246
1961	5.394
1962	5.547
1963	5.704
1964	5.865
1965	6.03
1966	6.199
1967	6.373
1968	6.804
1969	7.311
1970	7.852
1971	8.431
1972	9.048
1973	9.704
1974	10.403
1975	11.146
1976	11.937
1977	12.773
1978	13.659
1979	14.141
1980	14.555
1981	14.976
1982	15.409
1983	15.851
1984	16.305
1985	16.767
1986	17.241
1987	17.725
1988	18.221
1989	18.565
1990	18.884
1991	19.208
1992	19.536
1993	19.867
1994	20.203
1995	20.543
1996	20.888
1997	21.237
1998	21.59
1999	21.947
2000	22.309
2001	22.674
2002	23.044
2003	23.611
2004	24.223
2005	24.845
2006	25.478
2007	26.121
2008	26.776
2009	27.439
2010	28.114
2011	28.798
2012	29.493
2013	30.196
2014	30.904
2015	31.617
2016	32.333
2017	33.053