Year	Percent
1972	19.30372
1973	17.83946
1974	17.31576
1975	16.71656
1976	19.64228
1977	20.25626
1978	18.57421
1979	17.04468
1980	16.53618
1981	16.95093
1982	16.60467
1983	17.07602
1984	17.94385
1985	20.48214
1986	19.42057
1987	21.87861
1988	21.98235
1989	21.59203
1990	21.92919
1991	19.94841
1992	17.38701
1993	15.66765
1994	15.01519
1995	26.85274
1996	27.45342
1997	27.68037
1998	29.91329
1999	30.46705
2000	30.0217
2001	27.97819
2002	26.8527
2003	26.90131
2004	27.21739
2005	28.49947
2006	28.49235
2007	28.44886
2008	27.02637
2009	26.93829
2010	27.1107
2011	26.48745
2012	26.06863
2013	26.24819
2014	26.30097
2015	26.9858
2016	27.65461