Year	Percent
1960	16.14089
1961	16.21226
1962	16.25203
1963	16.29958
1964	16.34763
1965	16.39177
1966	16.4328
1967	16.46763
1968	16.50214
1969	16.5405
1970	16.59382
1971	16.53199
1972	16.39975
1973	16.27923
1974	16.17238
1975	16.07723
1976	15.99589
1977	15.92237
1978	15.85563
1979	15.79569
1980	15.73901
1981	15.74968
1982	15.79282
1983	15.84183
1984	15.8989
1985	15.95312
1986	15.99852
1987	16.02621
1988	16.04285
1989	16.07932
1990	16.12711
1991	16.22671
1992	16.34915
1993	16.46896
1994	16.59424
1995	16.72886
1996	16.86696
1997	16.99958
1998	17.12443
1999	17.19458
2000	17.23039
2001	17.29059
2002	17.37196
2003	17.4726
2004	17.57332
2005	17.70336
2006	17.85886
2007	17.99648
2008	18.11987
2009	18.24042
2010	18.3651
2011	18.43912
2012	18.46806
2013	18.51559
2014	18.57874
2015	18.66097
2016	18.77716
2017	18.88317