Year	Percent
1975	35.46013
1976	34.03024
1977	33.4726
1978	33.60495
1979	34.28669
1980	34.01817
1981	33.55909
1982	32.72163
1983	32.57376
1984	32.01989
1985	30.9016
1986	30.10041
1987	30.45859
1988	30.0727
1989	30.3809
1990	29.20338
1991	26.49203
1992	26.23391
1993	26.94407
1994	28.21259
1995	29.48577
1996	28.8448
1997	29.04333
1998	30.55108
1999	30.43184
2000	31.62974
2001	30.98444
2002	30.14721
2003	29.56145
2004	29.4232
2005	29.3028
2006	30.28839
2007	30.7454
2008	29.63848
2009	25.75112
2010	26.21219
2011	25.0141
2012	23.33515
2013	23.2534
2014	23.06569
2015	23.32637
2016	23.35742
2017	23.96856