Year	Percent
1981	25.87
1982	26.23
1983	25.83
1984	26.87
1985	26.84
1986	27.2
1987	27.68
1988	27.92
1989	28.8
1990	28.99
1991	24.02
1992	25.76
1993	20.28
1994	18.58
1996	15.06
1997	14.76
1998	14.62
1999	13.65
2000	13.89
2001	13.9
2002	14.44
2003	16.05
2004	16.25
2005	16
2006	18.19
2007	18.75
2008	19.67
2009	19.28
2010	18.65
2011	18.75
2012	19.28
2013	17.69
2014	17.9
2015	17.85
2016	17.14
2017	16.65