Year	Percent
1991	90.87
1992	90.91
1993	90.96
1994	90.99
1995	91.03
1996	91.07
1997	91.1
1998	91.14
1999	91.19
2000	91.23
2001	91.26
2002	91.3
2003	91.34
2004	91.39
2005	91.41
2006	91.46
2007	91.5
2008	91.53
2009	91.57
2010	91.61
2011	91.66
2012	91.69
2013	91.73
2014	91.77
2015	91.81