Year	Percent
1960	4.21138
1961	4.19328
1962	4.17319
1963	4.15414
1964	4.14002
1965	4.13257
1966	4.13255
1967	4.13849
1968	4.14913
1969	4.16205
1970	4.17625
1971	4.39136
1972	4.6336
1973	4.88803
1974	5.15535
1975	5.43466
1976	5.72592
1977	6.02801
1978	6.34548
1979	6.68166
1980	7.04041
1981	7.42247
1982	7.82992
1983	8.25798
1984	8.70081
1985	9.15079
1986	9.60836
1987	10.07586
1988	10.56223
1989	11.07565
1990	11.62891
1991	12.2267
1992	12.76748
1993	13.32428
1994	13.90844
1995	14.51512
1996	15.1453
1997	15.79939
1998	16.4891
1999	17.22354
2000	18.0132
2001	18.22
2002	18.44185
2003	18.68194
2004	18.93684
2005	19.20072
2006	19.47694
2007	19.76532
2008	20.06329
2009	20.36372
2010	20.66695
2011	20.96985
2012	21.27516
2013	21.59151
2014	21.93057
2015	22.30054
2016	22.70522
2017	23.14218