Year	Percent
1990	18.19
1991	18.13
1992	18.11
1993	18.06
1994	18.04
1995	17.95
1996	17.89
1997	17.85
1998	17.8
1999	17.75
2000	17.7
2001	17.66
2002	17.63
2003	17.62
2004	17.62
2005	17.64
2006	17.64
2007	17.65
2008	17.66
2009	17.69
2010	17.74
2011	17.78
2012	17.85
2013	17.92
2014	17.99
2015	18.05
2016	18.11
2017	18.16
2018	18.2
2019	18.24