Year	Percent
1960	3.48
1961	3.571
1962	3.612
1963	3.653
1964	3.695
1965	3.737
1966	3.78
1967	3.823
1968	3.867
1969	3.911
1970	3.956
1971	4.005
1972	4.198
1973	4.399
1974	4.61
1975	4.83
1976	5.061
1977	5.302
1978	5.553
1979	5.816
1980	6.091
1981	6.376
1982	6.616
1983	6.864
1984	7.121
1985	7.386
1986	7.66
1987	7.944
1988	8.238
1989	8.541
1990	8.854
1991	9.18
1992	9.582
1993	9.999
1994	10.433
1995	10.883
1996	11.35
1997	11.834
1998	12.336
1999	12.857
2000	13.397
2001	13.947
2002	14.24
2003	14.538
2004	14.841
2005	15.149
2006	15.462
2007	15.781
2008	16.105
2009	16.434
2010	16.768
2011	17.108
2012	17.458
2013	17.815
2014	18.182
2015	18.557
2016	18.942
2017	19.336