Year	Percent
1960	22.312
1961	22.553
1962	22.797
1963	23.042
1964	23.289
1965	23.537
1966	23.788
1967	24.04
1968	24.295
1969	24.551
1970	24.808
1971	25.068
1972	25.33
1973	25.593
1974	25.741
1975	25.858
1976	25.975
1977	26.092
1978	26.209
1979	26.327
1980	26.445
1981	26.417
1982	26.35
1983	26.283
1984	26.216
1985	26.149
1986	26.083
1987	26.016
1988	25.95
1989	25.884
1990	25.818
1991	25.752
1992	25.686
1993	25.62
1994	25.554
1995	25.068
1996	24.473
1997	23.889
1998	23.314
1999	22.749
2000	22.33
2001	22.33
2002	22.33
2003	22.33
2004	22.33
2005	22.33
2006	22.324
2007	22.317
2008	22.311
2009	22.305
2010	22.298
2011	22.305
2012	22.324
2013	22.355
2014	22.399
2015	22.456
2016	22.526
2017	22.608