Year	Percent
1960	21.46
1961	21.696
1962	21.934
1963	22.174
1964	22.416
1965	22.66
1966	22.905
1967	23.153
1968	23.403
1969	23.654
1970	23.907
1971	24.162
1972	24.419
1973	24.677
1974	24.938
1975	25.2
1976	25.465
1977	25.73
1978	25.998
1979	26.268
1980	26.54
1981	26.812
1982	27.087
1983	27.364
1984	27.643
1985	27.923
1986	28.205
1987	28.488
1988	28.774
1989	29.061
1990	29.349
1991	29.572
1992	29.369
1993	29.167
1994	28.965
1995	28.765
1996	28.565
1997	28.366
1998	28.168
1999	27.971
2000	27.774
2001	27.473
2002	26.329
2003	25.216
2004	24.133
2005	23.085
2006	22.068
2007	21.084
2008	20.131
2009	19.213
2010	18.45
2011	18.45
2012	18.45
2013	18.461
2014	18.482
2015	18.515
2016	18.558
2017	18.612