Year	Percent
1960	35.9453
1961	36.17208
1962	36.1944
1963	36.30961
1964	37.19298
1965	38.06818
1966	37.56374
1967	35.8805
1968	35.95263
1969	36.2256
1970	36.1432
1971	34.56646
1972	35.81435
1973	37.75209
1974	38.58057
1975	39.24289
1976	39.46087
1977	38.3066
1978	39.65751
1979	42.47718
1980	45.27759
1981	42.68132
1982	40.83473
1983	41.44676
1984	39.7944
1985	39.98071
1986	40.19016
1987	38.19418
1988	37.74869
1989	36.77414
1990	36.40688
1991	35.09591
1992	33.67638
1993	32.68973
1994	32.04382
1995	31.91711
1996	30.80974
1997	30.14121
1998	29.48712
1999	28.54485
2000	29.06573
2001	29.55043
2002	29.69304
2003	28.00773
2004	27.31361
2005	27.13741
2006	26.23562
2007	26.52626
2008	28.28426
2009	27.58362
2010	27.38134
2011	26.94254
2012	26.68106
2013	26.66809
2014	26.54514
2015	26.04273
2016	26.00914
2017	25.89867