Year	Percent
1960	4.5939
1961	4.7554
1962	4.92121
1963	5.09098
1964	5.2652
1965	5.44254
1966	5.62432
1967	5.81007
1968	6.0006
1969	6.19465
1970	6.39402
1971	6.59875
1972	6.80928
1973	7.00631
1974	7.1536
1975	7.29868
1976	7.44213
1977	7.58352
1978	7.72645
1979	7.87279
1980	8.02507
1981	8.18101
1982	8.34248
1983	8.58814
1984	8.96387
1985	9.36331
1986	9.79449
1987	10.25613
1988	10.73978
1989	11.22759
1990	11.71373
1991	12.19117
1992	12.6694
1993	13.02554
1994	12.89532
1995	12.81201
1996	12.77914
1997	12.78475
1998	12.81387
1999	12.84471
2000	12.86381
2001	12.8657
2002	12.85799
2003	12.8492
2004	12.85189
2005	12.87196
2006	12.91204
2007	12.96641
2008	13.02815
2009	13.08703
2010	13.1364
2011	13.17473
2012	13.20445
2013	13.22792
2014	13.24871
2015	13.26936
2016	13.29032
2017	13.31097