Year	Percent
1960	28.04625
1961	28.18782
1962	28.35459
1963	28.51969
1964	28.68302
1965	28.85195
1966	28.7238
1967	28.50873
1968	28.34613
1969	28.20017
1970	27.99939
1971	27.187
1972	26.09322
1973	25.03777
1974	24.05722
1975	23.15609
1976	22.5684
1977	22.25552
1978	21.94684
1979	21.6565
1980	21.3962
1981	21.21218
1982	21.08013
1983	20.9479
1984	20.8129
1985	20.65997
1986	20.5106
1987	20.38571
1988	20.27727
1989	20.19447
1990	20.13815
1991	20.08227
1992	20.03353
1993	20.00579
1994	20.01702
1995	20.03203
1996	20.05719
1997	20.1009
1998	20.12211
1999	20.14753
2000	20.17244
2001	20.19225
2002	20.28451
2003	20.45291
2004	20.6258
2005	20.79598
2006	20.95674
2007	21.09875
2008	21.21581
2009	21.34403
2010	21.49288
2011	21.6667
2012	21.86741
2013	22.05906
2014	22.23152
2015	22.36071
2016	22.47442
2017	22.60076