Year	Percent
1960	36.777
1961	36.849
1962	36.922
1963	36.995
1964	37.068
1965	37.141
1966	37.214
1967	37.287
1968	37.36
1969	37.433
1970	37.55
1971	37.809
1972	38.07
1973	38.331
1974	38.592
1975	38.855
1976	39.118
1977	39.381
1978	39.646
1979	39.91
1980	40.102
1981	39.828
1982	39.554
1983	39.28
1984	39.007
1985	38.736
1986	38.464
1987	38.194
1988	37.923
1989	37.654
1990	37.37
1991	37.006
1992	36.643
1993	36.283
1994	35.924
1995	35.566
1996	35.21
1997	34.856
1998	34.503
1999	34.152
2000	33.827
2001	33.629
2002	33.431
2003	33.234
2004	33.037
2005	32.841
2006	32.646
2007	32.451
2008	32.256
2009	32.063
2010	31.87
2011	31.7
2012	31.553
2013	31.429
2014	31.328
2015	31.249
2016	31.193
2017	31.159