Year	Percent
1960	18.926
1961	18.986
1962	19.061
1963	19.141
1964	19.221
1965	19.301
1966	19.382
1967	19.593
1968	19.844
1969	20.097
1970	20.353
1971	20.611
1972	20.752
1973	20.832
1974	20.912
1975	20.992
1976	21.073
1977	21.113
1978	21.133
1979	21.153
1980	21.173
1981	21.193
1982	21.2
1983	21.2
1984	21.2
1985	21.2
1986	21.2
1987	21.2
1988	21.2
1989	21.2
1990	21.2
1991	21.2
1992	21.258
1993	21.347
1994	21.436
1995	21.526
1996	21.616
1997	21.705
1998	21.796
1999	21.886
2000	21.977
2001	22.068
2002	21.941
2003	21.698
2004	21.457
2005	21.218
2006	20.981
2007	20.75
2008	20.524
2009	20.3
2010	20.078
2011	19.857
2012	19.624
2013	19.385
2014	19.149
2015	18.914
2016	18.681
2017	18.452