Year	Percent
1990	82.06
1991	81.88
1992	81.74
1993	81.94
1994	81.81
1995	81.79
1996	81.7
1997	81.59
1998	81.49
1999	81.51
2000	81.54
2001	81.35
2002	81.24
2003	81.07
2004	80.71
2005	80.55
2006	80.57
2007	80.58
2008	80.63
2009	80.68
2010	80.62
2011	80.71
2012	80.69
2013	80.74
2014	80.79
2015	80.91
2016	81.14
2017	81.34
2018	81.47
2019	81.59