Year	Percent
1990	20.53
1991	20.6
1992	20.68
1993	20.76
1994	20.83
1995	20.91
1996	20.99
1997	21.07
1998	21.14
1999	21.22
2000	21.3
2001	21.35
2002	21.41
2003	21.46
2004	21.52
2005	21.58
2006	21.63
2007	21.68
2008	21.73
2009	21.78
2010	21.83
2011	22.23
2012	22.64
2013	23.04
2014	23.45
2015	23.85