Year	Percent
1981	36.2995
1982	33.72285
1990	30.24828
1991	28.04574
1992	31.40549
1993	28.92772
1994	24.80913
1995	23.57935
1996	23.36721
1997	21.48032
1998	18.4989
1999	15.86913
2000	19.82041
2001	22.62185
2002	22.45469
2003	22.7214
2004	24.28622
2005	23.87004
2006	27.23208
2007	31.11698
2008	33.15431
2009	26.64559
2010	27.07708
2011	28.00493
2012	26.8441
2013	25.56261
2014	24.6911
2015	25.16818
2016	23.96905
2017	24.40835