Year	Percent
1960	31.62687
1961	33.19653
1962	28.47167
1963	30.87661
1964	30.74385
1965	33.59247
1966	32.66782
1967	32.97586
1968	33.04184
1969	33.69013
1970	33.03024
1971	32.15066
1972	30.47692
1973	28.43671
1974	30.62217
1975	26.8495
1976	26.46942
1977	27.00781
1978	25.85778
1979	27.89605
1980	27.13148
1981	28.72489
1982	29.88324
1983	25.34239
1984	26.67573
1985	28.07186
1986	28.55644
1987	27.41924
1988	28.02288
1989	29.66523
1990	28.99706
1991	24.26599
1992	22.39021
1993	23.64785
1994	24.3169
1995	26.04703
1996	24.85126
1997	24.91667
1998	25.691
1999	26.22874
2000	26.31999
2001	23.48978
2002	24.50318
2003	26.02707
2004	27.14519
2005	27.52596
2006	27.59744
2007	27.51218
2008	28.60648
2009	27.32508
2010	26.76627
2011	26.41416
2012	27.64972
2013	27.80951
2014	26.65061
2015	26.14333
2016	25.29549
2017	24.17924