Year	Percent
1960	20.437
1961	20.315
1962	20.102
1963	19.89
1964	19.679
1965	19.471
1966	19.264
1967	19.059
1968	18.855
1969	18.654
1970	18.453
1971	18.364
1972	18.354
1973	18.344
1974	18.334
1975	18.324
1976	18.314
1977	18.304
1978	18.294
1979	18.284
1980	18.274
1981	18.187
1982	18.043
1983	17.901
1984	17.759
1985	17.619
1986	17.479
1987	17.34
1988	17.201
1989	17.064
1990	16.928
1991	16.821
1992	16.736
1993	16.651
1994	16.566
1995	16.482
1996	16.29
1997	15.994
1998	15.702
1999	15.414
2000	15.13
2001	14.962
2002	14.906
2003	14.85
2004	14.795
2005	14.739
2006	14.684
2007	14.629
2008	14.574
2009	14.519
2010	14.464
2011	14.41
2012	14.367
2013	14.334
2014	14.313
2015	14.303
2016	14.303
2017	14.315