Year	Percent
1990	19.35
1991	19.34
1992	19.38
1993	19.49
1994	19.57
1995	19.53
1996	19.76
1997	19.93
1998	20.02
1999	20.03
2000	20.03
2001	19.81
2002	19.52
2003	19.22
2004	18.93
2005	18.64
2006	18.91
2007	19.16
2008	19.35
2009	19.4
2010	19.28
2011	18.71
2012	18.02
2013	17.6
2014	17.2
2015	16.84
2016	16.53
2017	16.24
2018	15.83
2019	15.42