Year	Percent
1990	25.59
1991	25.83
1992	26.06
1993	26.3
1994	26.53
1995	26.76
1996	27
1997	27.23
1998	27.47
1999	27.7
2000	27.94
2001	28.17
2002	28.4
2003	28.64
2004	28.87
2005	29.11
2006	29.34
2007	29.58
2008	29.81
2009	30.04
2010	30.28
2011	30.51
2012	30.75
2013	30.98
2014	31.22
2015	31.45