Year	Percent
1972	21.83384
1973	22.14278
1974	21.97095
1975	21.27015
1976	23.63663
1977	22.50608
1978	22.08293
1979	21.93797
1980	24.1979
1981	25.28395
1982	26.35286
1983	27.50232
1984	28.08581
1985	27.17304
1986	27.32082
1987	27.41522
1988	28.71687
1989	25.79826
1990	25.32958
1991	25.59857
1992	25.90137
1993	25.80962
1994	27.02414
1995	26.49554
1996	27.17919
1997	26.88281
1998	26.52934
1999	26.69699
2000	26.37731
2001	24.23483
2002	23.51812
2003	24.16379
2004	25.08659
2005	25.4753
2006	26.8077
2007	25.91965
2008	23.7393
2009	21.98629
2010	21.81543
2011	21.82224
2012	22.47764
2013	22.80055
2014	23.16473
2015	18.86715
2016	18.78236