Year	Percent
1971	34.09156
1972	32.77228
1973	32.74223
1974	35.09752
1975	33.21745
1976	34.12539
1977	34.14821
1978	33.863
1979	34.03095
1980	33.74604
1981	35.33829
1982	35.37067
1983	34.55292
1984	34.07585
1985	31.69602
1986	30.04159
1987	27.40191
1988	26.33233
1989	26.35359
1990	24.92418
1991	24.49925
1992	24.44547
1993	24.62349
1994	25.39934
1995	24.60228
1996	23.82667
1997	23.62089
1998	22.74238
1999	23.00029
2000	22.43696
2001	22.00416
2002	22.76577
2003	22.34156
2004	22.56111
2005	22.83733
2006	21.78872
2007	20.9183
2008	20.98124
2009	19.29617
2010	19.13495
2011	19.13281
2012	19.25443
2013	18.80849
2014	19.97044
2015	20.4402