Year	Percent
1976	30.6747
1977	29.82692
1978	30.60011
1979	31.08668
1980	20.64456
1981	25.14658
1982	18.09031
1983	17.39148
1984	21.94818
1985	23.37035
1986	21.83719
1987	25.48537
1988	30.88861
1989	30.98721
1990	30.21045
1991	28.22176
1992	29.06688
1993	30.53955
1994	31.77928
1995	25.58049
1996	24.67869
1997	29.38878
1998	25.77861
1999	26.77398
2000	26.13778
2001	20.69684
2002	22.09233
2003	23.59944
2004	24.42094
2005	22.5295
2006	25.5732
2007	25.98556
2008	25.34599
2009	23.76575
2010	27.10541
2011	23.94638
2012	24.38057
2013	22.03213
2014	19.67571
2015	18.09106
2016	17.90093
2017	16.89279