Year	Percent
1960	2.37474
1961	2.59012
1962	2.8246
1963	2.9757
1964	3.13476
1965	3.29961
1966	3.47518
1967	3.66134
1968	3.857
1969	4.05862
1970	4.26559
1971	4.47614
1972	4.69188
1973	4.99041
1974	5.3124
1975	5.66291
1976	6.04612
1977	6.4602
1978	6.90615
1979	7.0411
1980	7.17176
1981	7.29631
1982	7.41589
1983	7.52733
1984	7.62805
1985	7.71516
1986	7.78857
1987	7.84941
1988	7.90271
1989	7.95311
1990	8.00561
1991	8.06072
1992	8.11727
1993	8.16715
1994	8.1974
1995	8.22201
1996	8.24241
1997	8.25823
1998	8.26905
1999	8.27254
2000	8.26866
2001	8.25587
2002	8.23809
2003	8.22069
2004	8.21085
2005	8.21118
2006	8.22413
2007	8.24702
2008	8.27632
2009	8.30572
2010	8.33293
2011	8.35739
2012	8.38093
2013	8.40592
2014	8.43537
2015	8.47117
2016	8.51366
2017	8.56192