Year	Percent
1979	21.95122
1980	17.68729
1981	18.52897
1982	17.11874
1983	23.54908
1984	25.09591
1985	26.09971
1986	22.46585
1987	19.10574
1988	18.78326
1989	19.02977
1990	19.58116
1991	15.5348
1992	15.19676
1993	16.9552
1994	15.27717
1995	15.42589
1996	16.51027
1997	15.70904
1998	26.09801
1999	24.84607
2000	22.96413
2001	23.52988
2002	24.26613
2003	22.82889
2004	23.59875
2005	22.2745
2006	20.98496
2007	20.9592
2008	20.01403
2009	19.39074
2010	19.54274
2011	20.85915
2012	21.6187
2013	21.52123
2014	23.39603
2015	22.42113
2016	21.68298
2017	21.37023
2018	21.21919