Year	Percent
1980	24.10783
1981	26.04977
1982	20.75314
1983	20.21164
1984	23.54196
1985	21.61128
1986	20.36427
1987	21.93093
1988	25.37313
1989	30.29051
1990	25.65065
1991	29.56912
1992	26.62864
1993	28.48042
1994	20.55196
1995	21.70731
1996	20.35223
1997	20.17069
1998	19.0695
1999	24.58649
2000	31.70899
2001	24.78681
2002	23.83233
2003	20.49317
2004	19.55266
2005	20.91929
2006	19.3077
2007	19.60808
2008	25.59283
2009	19.07551
2010	13.22337
2011	17.01863
2012	16.00498
2013	18.28642
2014	21.3433
2015	22.47396
2016	22.06526
2017	19.01523