Year	Percent
1960	11.26331
1961	11.47357
1962	11.68209
1963	11.8909
1964	12.10568
1965	12.32786
1966	12.56193
1967	12.80775
1968	13.06638
1969	13.33476
1970	13.61576
1971	13.90797
1972	14.00698
1973	14.11562
1974	14.23771
1975	14.37431
1976	14.52595
1977	14.69114
1978	14.87454
1979	15.07872
1980	15.3064
1981	15.55659
1982	15.83005
1983	16.12255
1984	16.42965
1985	16.74522
1986	17.07143
1987	17.40803
1988	17.75309
1989	18.10076
1990	18.45165
1991	18.80329
1992	19.15886
1993	19.08237
1994	18.88181
1995	18.70231
1996	18.54467
1997	18.40634
1998	18.28627
1999	18.18233
2000	18.09265
2001	18.01674
2002	17.95403
2003	17.90146
2004	17.85551
2005	17.8131
2006	17.7732
2007	17.73527
2008	17.69833
2009	17.66124
2010	17.62358
2011	17.58467
2012	17.54415
2013	17.50162
2014	17.45717
2015	17.41083
2016	17.36249
2017	17.31221