Year	Percent
1990	31.82
1991	31.78
1992	31.75
1993	31.72
1994	31.68
1995	31.65
1996	31.61
1997	31.58
1998	31.55
1999	31.51
2000	31.48
2001	31.45
2002	31.41
2003	31.38
2004	31.35
2005	31.32
2006	31.28
2007	31.24
2008	31.2
2009	31.17
2010	31.13
2011	31.09
2012	31.06
2013	31.02
2014	30.99
2015	30.95