Year	Percent
1970	28.16973
1971	25.89776
1972	27.13012
1973	28.50677
1974	31.63306
1975	29.90746
1976	28.22845
1977	26.1894
1978	24.01028
1979	23.40653
1980	24.2097
1981	22.55554
1982	22.43257
1983	21.58363
1984	20.4588
1985	20.68067
1986	21.58363
1987	23.0061
1988	25.17964
1989	26.498
1990	26.65731
1991	25.84361
1992	23.90553
1993	21.32045
1994	21.50715
1995	22.36876
1996	22.08212
1997	22.4832
1998	23.79964
1999	25.42486
2000	26.5795
2001	26.57306
2002	26.9006
2003	27.82337
2004	28.75287
2005	30.01571
2006	31.29614
2007	31.33547
2008	29.59547
2009	24.56536
2010	23.54896
2011	21.90735
2012	19.99394
2013	18.71135
2014	19.45212
2015	20.39143
2016	20.48784
2017	21.14506