Year	Percent
1960	2.6
1961	2.654
1962	2.71
1963	2.766
1964	2.824
1965	2.882
1966	2.942
1967	3.003
1968	3.066
1969	3.129
1970	3.194
1971	3.336
1972	3.491
1973	3.652
1974	3.821
1975	3.998
1976	4.182
1977	4.374
1978	4.575
1979	4.656
1980	4.721
1981	4.786
1982	4.853
1983	4.92
1984	4.988
1985	5.057
1986	5.127
1987	5.198
1988	5.27
1989	5.342
1990	5.416
1991	5.491
1992	6.288
1993	7.313
1994	8.49
1995	9.837
1996	11.374
1997	12.466
1998	13.244
1999	14.064
2000	14.926
2001	15.829
2002	16.777
2003	16.904
2004	16.908
2005	16.912
2006	16.917
2007	16.921
2008	16.925
2009	16.93
2010	16.934
2011	16.938
2012	16.943
2013	16.947
2014	16.967
2015	17.004
2016	17.056
2017	17.125