Year	Percent
1970	41.40168
1971	32.49618
1972	26.1458
1973	32.06941
1974	46.53708
1975	58.03147
1976	55.3969
1977	61.24104
1978	51.93137
1979	50.6116
1980	60.67427
1981	59.30943
1982	59.92791
1983	61.44128
1984	59.81851
1985	60.93238
1986	63.85963
1987	51.61035
1988	47.69657
1989	47.65857
1990	33.86229
1991	22.20709
1992	14.03613
1993	11.17705
1994	12.0921
1995	14.54166
1996	20.86556
1997	22.86113
1998	22.18265
1999	20.66307
2000	21.32804
2001	20.51612
2002	16.66245
2003	17.71974
2004	20.80282
2005	24.39492
2006	25.43793
2007	25.36778
2008	31.79063
2009	20.47499
2010	25.50952
2011	29.97911
2012	27.02766
2013	25.91124
2014	22.18555
2015	17.27478