Year	Percent
1975	29.56
1976	32.6
1978	35.23
1980	61.97
1981	37.95
1982	39.42
1985	56.01
1988	36.92
1990	23.08
1991	22.82
1992	22.2
1993	22.4
1994	21.73
1995	22.51
1996	23.04
1997	23.41
1998	22.09
1999	22.82
2000	22.276
2001	20.93
2002	19.5558
2003	16.71
2004	15.6
2005	15.7989
2006	14.4568
2007	14.2832
2008	13.83
2009	14.8352
2010	16.2076
2011	17.2
2012	18.0845
2013	20.0714
2014	18.14
2015	17.8574
2016	17.3969
2017	15.3886