Year	Percent
1990	18.44
1991	18.4
1992	18.33
1993	18.22
1994	18.09
1995	17.92
1996	17.84
1997	17.76
1998	17.7
1999	17.64
2000	17.6
2001	17.84
2002	18.12
2003	18.39
2004	18.68
2005	18.95
2006	18.96
2007	18.95
2008	18.92
2009	18.9
2010	18.87
2011	18.9
2012	18.91
2013	18.92
2014	18.93
2015	18.95
2016	19.03
2017	19.1
2018	19.18
2019	19.23