Year	Percent
1990	27.41
1991	28
1992	28.59
1993	29.18
1994	29.77
1995	30.35
1996	30.94
1997	31.53
1998	32.12
1999	32.71
2000	33.3
2001	32.94
2002	32.58
2003	32.23
2004	31.87
2005	31.51
2006	31.57
2007	31.63
2008	31.69
2009	31.75
2010	31.81
2011	31.86
2012	31.92
2013	31.98
2014	32.04
2015	32.1