Year	Percent
1960	39.656
1961	39.04
1962	38.427
1963	37.817
1964	37.211
1965	36.61
1966	36.012
1967	35.418
1968	34.829
1969	34.245
1970	33.819
1971	33.898
1972	33.978
1973	34.057
1974	34.137
1975	34.217
1976	34.297
1977	34.377
1978	34.457
1979	34.538
1980	34.618
1981	34.698
1982	34.779
1983	34.86
1984	34.94
1985	35.021
1986	35.102
1987	35.183
1988	35.264
1989	35.345
1990	35.426
1991	35.465
1992	35.085
1993	34.709
1994	34.334
1995	33.961
1996	33.59
1997	33.221
1998	32.855
1999	32.49
2000	32.127
2001	31.74
2002	31.098
2003	30.464
2004	29.837
2005	29.218
2006	28.606
2007	28.002
2008	27.405
2009	26.818
2010	26.238
2011	25.703
2012	25.526
2013	25.35
2014	25.175
2015	25
2016	24.846
2017	24.713