Year	Percent
1960	2.077
1961	2.115
1962	2.154
1963	2.193
1964	2.233
1965	2.294
1966	2.395
1967	2.501
1968	2.611
1969	2.726
1970	2.845
1971	2.97
1972	3.101
1973	3.236
1974	3.377
1975	3.525
1976	3.678
1977	3.838
1978	4.005
1979	4.178
1980	4.339
1981	4.503
1982	4.674
1983	4.85
1984	5.033
1985	5.221
1986	5.417
1987	5.62
1988	5.83
1989	6.047
1990	6.271
1991	6.455
1992	6.637
1993	6.823
1994	7.014
1995	7.211
1996	7.412
1997	7.618
1998	7.83
1999	8.036
2000	8.246
2001	8.461
2002	8.682
2003	8.908
2004	9.139
2005	9.375
2006	9.617
2007	9.864
2008	10.118
2009	10.376
2010	10.642
2011	10.915
2012	11.194
2013	11.482
2014	11.776
2015	12.078
2016	12.388
2017	12.706