Year	Percent
1960	38.7
1961	38.436
1962	38.172
1963	37.909
1964	37.646
1965	37.385
1966	37.124
1967	36.864
1968	36.604
1969	36.346
1970	36.088
1971	35.803
1972	35.394
1973	34.989
1974	34.585
1975	34.183
1976	33.783
1977	33.387
1978	32.992
1979	32.599
1980	32.209
1981	31.909
1982	31.89
1983	31.871
1984	31.851
1985	31.832
1986	31.813
1987	31.707
1988	31.602
1989	31.497
1990	31.392
1991	31.262
1992	31.076
1993	30.891
1994	30.706
1995	30.522
1996	30.4
1997	30.416
1998	30.432
1999	30.449
2000	30.465
2001	30.495
2002	30.557
2003	30.619
2004	30.681
2005	30.743
2006	30.805
2007	30.867
2008	30.929
2009	30.992
2010	31.054
2011	31.091
2012	31.059
2013	31.027
2014	30.995
2015	30.962
2016	30.93
2017	30.914