Year	Percent
1960	18.14028
1961	18.18939
1962	18.26263
1963	18.34764
1964	18.42301
1965	18.5054
1966	18.58372
1967	18.64932
1968	18.7133
1969	18.77066
1970	18.82348
1971	18.88187
1972	18.93834
1973	18.99008
1974	19.01851
1975	19.02
1976	19.02886
1977	19.05371
1978	19.10159
1979	19.18106
1980	19.20624
1981	19.16484
1982	19.13603
1983	19.12472
1984	19.12289
1985	19.11769
1986	19.11027
1987	19.10233
1988	19.09157
1989	19.2603
1990	19.32919
1991	19.10915
1992	18.89764
1993	18.70307
1994	18.51445
1995	18.32815
1996	18.14934
1997	17.97819
1998	17.81392
1999	17.66009
2000	17.5032
2001	17.40749
2002	17.41405
2003	17.4209
2004	17.41637
2005	17.40803
2006	17.39213
2007	17.37611
2008	17.36353
2009	17.3476
2010	17.34394
2011	17.35022
2012	17.46304
2013	17.55651
2014	17.64949
2015	17.73737
2016	17.83596
2017	17.94226