Year	Percent
1960	33.10134
1961	25.11925
1962	29.31446
1963	30.64942
1964	29.99535
1965	25.80769
1966	23.9759
1967	24.2002
1968	10.20433
1969	12.10339
1970	12.20501
1971	10.36948
1972	25.00443
1973	34.57407
1974	38.74397
1975	28.79603
1976	26.21797
1977	25.1154
1978	30.82654
1979	35.31427
1980	33.63488
1981	30.29828
1982	25.58747
1983	29.29748
1984	27.00002
1985	27.70917
1986	27.45534
1987	29.96564
1988	28.88279
1989	29.50386
1990	32.55652
1991	33.08548
1992	34.48944
1993	31.59583
1994	31.46945
1995	32.23167
1996	34.03973
1997	32.68768
1998	28.69181
1999	29.58529
2000	26.89839
2001	26.86913
2002	29.24482
2003	29.99346
2004	31.9746
2005	37.81507
2006	34.06208
2007	33.09201
2008	36.83708
2009	29.25398
2010	33.37886
2011	32.48993
2012	30.60469
2013	30.17392
2014	29.52187
2015	24.3514
2016	24.12377
2017	26.00903