Year	Percent
1960	33.168
1961	33.56
1962	33.955
1963	34.351
1964	34.751
1965	35.152
1966	35.555
1967	35.96
1968	36.368
1969	36.777
1970	36.876
1971	36.607
1972	36.337
1973	36.07
1974	35.802
1975	35.536
1976	35.27
1977	35.006
1978	34.743
1979	34.502
1980	34.289
1981	34.078
1982	33.866
1983	33.655
1984	33.445
1985	33.235
1986	33.026
1987	32.818
1988	32.61
1989	32.231
1990	31.658
1991	31.09
1992	30.527
1993	29.972
1994	29.421
1995	28.877
1996	28.338
1997	27.806
1998	27.28
1999	26.76
2000	26.501
2001	26.503
2002	26.505
2003	26.507
2004	26.508
2005	26.51
2006	26.512
2007	26.514
2008	26.516
2009	26.518
2010	26.52
2011	26.521
2012	26.545
2013	26.589
2014	26.655
2015	26.742
2016	26.851
2017	26.982