Year	Percent
1960	4.40601
1961	4.60765
1962	4.81698
1963	4.98608
1964	5.15887
1965	5.48129
1966	5.82275
1967	6.18349
1968	6.56533
1969	6.96548
1970	7.36945
1971	7.79412
1972	8.24133
1973	8.70897
1974	9.2005
1975	9.71569
1976	10.20027
1977	10.50236
1978	10.80998
1979	11.1219
1980	11.43904
1981	11.75934
1982	12.08655
1983	12.42043
1984	12.76275
1985	13.11159
1986	13.4707
1987	13.81288
1988	14.08278
1989	14.36349
1990	14.66085
1991	14.97449
1992	15.30548
1993	15.65212
1994	16.0191
1995	16.40611
1996	16.81528
1997	17.24201
1998	17.68581
1999	18.14007
2000	18.60198
2001	19.06669
2002	19.53881
2003	20.01687
2004	20.5026
2005	20.99227
2006	21.49021
2007	21.99577
2008	22.51127
2009	23.0397
2010	23.58364
2011	24.14435
2012	24.72262
2013	25.31995
2014	25.9378
2015	26.57749
2016	27.24034
2017	27.92746