Year	Percent
1972	17.88879
1973	18.19695
1974	18.39734
1975	18.01093
1976	17.86233
1977	17.99929
1978	18.77755
1979	18.55012
1980	18.578
1981	18.94983
1982	18.19042
1983	18.03259
1984	18.96408
1985	18.10378
1986	17.97234
1987	17.72008
1988	18.5628
1989	17.80575
1990	17.98108
1991	18.16718
1992	18.92048
1993	18.93546
1994	18.34557
1995	24.60011
1996	25.94004
1997	26.79771
1998	26.88985
1999	26.62782
2000	26.22508
2001	27.96557
2002	26.9755
2003	26.64697
2004	26.30867
2005	25.51396
2006	25.05044
2007	25.39199
2008	26.07752
2009	25.22517
2010	25.28164
2011	25.42799
2012	25.9282
2013	26.41517
2014	26.42452
2015	26.77939
2016	25.66657