Year	Percent
1960	4.417
1961	4.617
1962	4.826
1963	5.045
1964	5.273
1965	5.509
1966	5.757
1967	6.014
1968	6.283
1969	6.562
1970	6.664
1971	6.737
1972	6.812
1973	6.887
1974	6.963
1975	7.04
1976	7.118
1977	7.197
1978	7.276
1979	7.356
1980	7.534
1981	7.834
1982	8.147
1983	8.47
1984	8.805
1985	9.152
1986	9.511
1987	9.882
1988	10.267
1989	10.665
1990	11.076
1991	11.454
1992	11.79
1993	12.133
1994	12.485
1995	12.846
1996	13.216
1997	13.594
1998	13.982
1999	14.379
2000	14.786
2001	15.201
2002	15.626
2003	16.061
2004	16.507
2005	16.96
2006	17.425
2007	17.899
2008	18.384
2009	18.878
2010	19.383
2011	19.898
2012	20.424
2013	20.958
2014	21.504
2015	22.06
2016	22.624
2017	23.196