Year	Percent
1960	29.009
1961	29.049
1962	29.089
1963	29.129
1964	29.169
1965	29.209
1966	29.249
1967	29.289
1968	29.329
1969	29.369
1970	29.425
1971	29.533
1972	29.641
1973	29.75
1974	29.858
1975	29.967
1976	30.076
1977	30.186
1978	30.295
1979	30.405
1980	30.488
1981	30.396
1982	30.305
1983	30.215
1984	30.124
1985	30.033
1986	29.943
1987	29.853
1988	29.763
1989	29.673
1990	29.583
1991	29.493
1992	29.404
1993	29.315
1994	29.226
1995	29.137
1996	29.048
1997	28.959
1998	28.871
1999	28.782
2000	28.694
2001	28.606
2002	28.518
2003	28.307
2004	28.064
2005	27.822
2006	27.582
2007	27.343
2008	27.105
2009	26.869
2010	26.634
2011	26.4
2012	26.4
2013	26.4
2014	26.414
2015	26.441
2016	26.482
2017	26.538