Year	Percent
1960	3.40801
1961	3.58477
1962	3.77014
1963	3.96391
1964	4.26023
1965	4.6786
1966	5.12816
1967	5.608
1968	6.13676
1969	6.7404
1970	7.44451
1971	8.29822
1972	9.30794
1973	10.35893
1974	11.24363
1975	11.46764
1976	10.82717
1977	10.04002
1978	9.33637
1979	8.8668
1980	8.66418
1981	8.70535
1982	8.93518
1983	9.61654
1984	10.3754
1985	11.00208
1986	11.58028
1987	12.13026
1988	12.68274
1989	13.28917
1990	13.99486
1991	14.82041
1992	14.89308
1993	14.96867
1994	14.97989
1995	14.88167
1996	14.66323
1997	14.35347
1998	13.99671
1999	13.64416
2000	13.32595
2001	13.04513
2002	13.17277
2003	13.31404
2004	13.45774
2005	13.59383
2006	13.72412
2007	9.96692
2008	10.36947
2009	10.78796
2010	11.22772
2011	11.68909
2012	11.96336
2013	12.24313
2014	12.52889
2015	12.81837
2016	13.11152
2017	13.40902