Year	Percent
1960	70.924
1961	71.243
1962	71.539
1963	71.834
1964	72.127
1965	72.417
1966	72.706
1967	72.993
1968	73.279
1969	73.561
1970	73.843
1971	73.992
1972	74.046
1973	74.1
1974	74.154
1975	74.208
1976	74.262
1977	74.316
1978	74.37
1979	74.424
1980	74.477
1981	74.466
1982	74.406
1983	74.346
1984	74.286
1985	74.227
1986	74.167
1987	74.107
1988	74.046
1989	73.986
1990	73.926
1991	73.869
1992	73.815
1993	73.762
1994	73.708
1995	73.654
1996	73.6
1997	73.546
1998	73.492
1999	73.438
2000	73.383
2001	73.375
2002	73.401
2003	73.427
2004	73.452
2005	73.478
2006	73.504
2007	73.53
2008	73.556
2009	73.581
2010	73.607
2011	73.631
2012	73.653
2013	73.674
2014	73.696
2015	73.718
2016	73.739
2017	73.761