Year	Percent
1990	20.25
1991	20.29
1992	20.33
1993	20.36
1994	20.4
1995	20.44
1996	20.48
1997	20.52
1998	20.56
1999	20.6
2000	20.64
2001	20.28
2002	19.91
2003	19.55
2004	19.18
2005	18.82
2006	18.84
2007	18.86
2008	18.88
2009	18.9
2010	18.92
2011	18.94
2012	18.96
2013	18.98
2014	19
2015	19.01