Year	Percent
1990	81.9
1991	81.67
1992	81.27
1993	80.7
1994	80.56
1995	80.81
1996	80.6
1997	80.87
1998	81.13
1999	81.15
2000	81.33
2001	79.43
2002	77.37
2003	75.18
2004	72.85
2005	70.38
2006	67.79
2007	65.09
2008	62.28
2009	59.39
2010	56.43
2011	56.48
2012	56.43
2013	56.4
2014	56.08
2015	55.87
2016	55.82
2017	55.78
2018	55.88
2019	55.76