Year	Percent
1975	22.52918
1976	26.56483
1977	34.88537
1978	37.21619
1979	38.72658
1980	37.81402
1981	33.81278
1982	31.72017
1983	30.11349
1984	33.59503
1985	30.34003
1986	25.92292
1987	25.53084
1988	27.45629
1989	30.89193
1990	27.04543
1991	25.84215
1992	28.69896
1993	24.0877
1994	25.51565
1995	26.59128
1996	24.25982
1997	24.08904
1998	19.08222
1999	19.94187
2000	20.57939
2001	17.89466
2002	20.46098
2003	19.6723
2004	22.07004
2005	22.45066
2006	24.60557
2007	24.38008
2008	28.94349
2009	22.86609
2010	23.82652
2011	18.81395
2012	16.12134
2013	13.18541
2014	12.34473
2015	14.21993
2016	16.78349
2017	20.44364