Year	Percent
1960	28.21752
1961	27.67853
1962	27.69962
1963	26.01177
1964	26.77049
1965	26.37307
1966	26.44346
1967	27.74151
1968	27.68332
1969	28.25874
1970	28.25355
1971	28.47639
1972	28.74853
1973	30.20574
1974	31.82214
1975	28.77651
1976	31.21129
1977	61.84362
1978	34.02698
1979	35.83429
1991	25.62232
1992	26.15315
1993	27.45304
1994	29.15124
1995	29.09789
1996	28.86967
1997	29.35154
1998	28.47478
1999	28.60137
2000	29.03413
2001	29.03516
2002	29.44403
2003	29.88935
2004	32.6501
2005	34.35667
2006	37.45775
2007	37.69898
2008	36.32693
2009	33.55168
2010	35.77201
2011	37.2361
2012	35.53355
2013	33.85252
2014	31.64952
2015	29.76412
2016	30.00032