Year	Percent
1972	25.65126
1973	24.28953
1974	30.61053
1975	30.39415
1976	33.76848
1977	35.12672
1978	33.19172
1979	37.75831
1980	35.88003
1981	34.37591
1982	38.98409
1983	38.9657
1984	37.42893
1985	35.16084
1986	34.19282
1987	31.47435
1988	29.72039
1989	27.87261
1990	26.85211
1991	20.12304
1992	26.66255
1993	27.02001
1994	27.4966
1995	27.04794
1996	26.34557
1997	26.62004
1998	25.68564
1999	25.55103
2000	27.12283
2001	26.68215
2002	25.72504
2003	24.98244
2004	24.84634
2005	25.36245
2006	26.08557
2007	26.05637
2008	23.78697
2009	21.85841
2010	22.53328
2011	22.72831
2012	22.12153
2013	22.95054
2014	23.29141
2015	23.37929
2016	23.37306