Year	Percent
1981	18.55
1985	18.78
1990	20.57
1991	25.68
1992	19.4
1993	20.11
1994	19.5
1995	23.43
1996	22.41
1997	23.48
1998	22.5
2002	22.38
2003	22.98
2004	35.68
2005	25.65
2006	26.6
2007	26.61
2008	35.75
2009	25.13
2010	24.22
2012	26.07
2013	26.16
2014	26.47
2015	26.01
2016	26.39
2017	28.4