Year	Percent
1960	11.19029
1961	11.51005
1962	11.84795
1963	12.20756
1964	12.60984
1965	13.02177
1966	13.44662
1967	13.88445
1968	14.3359
1969	14.79692
1970	15.27107
1971	15.67019
1972	15.71751
1973	15.76228
1974	15.80723
1975	15.85242
1976	15.89928
1977	15.94744
1978	16.00236
1979	16.06702
1980	16.14488
1981	16.23379
1982	16.33608
1983	16.45309
1984	16.58815
1985	16.74003
1986	16.91298
1987	17.10481
1988	17.31034
1989	17.51923
1990	17.72895
1991	17.93707
1992	18.1477
1993	18.36438
1994	18.5979
1995	18.75434
1996	18.49317
1997	18.25214
1998	18.03079
1999	17.82793
2000	17.64225
2001	17.47327
2002	17.31963
2003	17.17657
2004	17.03881
2005	16.92074
2006	17.03422
2007	17.1482
2008	17.2648
2009	17.38514
2010	17.28778
2011	17.19673
2012	17.11114
2013	17.02989
2014	16.95255
2015	16.87831
2016	16.80706
2017	16.73879