Year	Percent
1960	27.644
1961	28.269
1962	28.903
1963	29.545
1964	30.197
1965	30.855
1966	31.521
1967	32.196
1968	32.878
1969	33.567
1970	34.141
1971	34.317
1972	34.495
1973	34.672
1974	34.849
1975	35.027
1976	35.206
1977	35.385
1978	35.564
1979	35.744
1980	35.882
1981	35.754
1982	35.626
1983	35.498
1984	35.37
1985	35.243
1986	35.116
1987	34.989
1988	34.862
1989	34.735
1990	34.609
1991	34.474
1992	34.283
1993	34.093
1994	33.904
1995	33.715
1996	33.526
1997	33.339
1998	33.151
1999	32.965
2000	32.778
2001	32.598
2002	32.455
2003	32.312
2004	32.17
2005	32.028
2006	31.886
2007	31.744
2008	31.603
2009	31.463
2010	31.322
2011	31.182
2012	31.062
2013	30.964
2014	30.885
2015	30.827
2016	30.79
2017	30.773