Year	Percent
1990	23.17
1991	23.15
1992	23.16
1993	23.18
1994	21.85
1995	20.54
1996	19.31
1997	18.13
1998	17
1999	15.93
2000	14.91
2001	13.91
2002	12.97
2003	13.15
2004	13.33
2005	13.5
2006	13.73
2007	13.95
2008	14.16
2009	14.4
2010	14.68
2011	14.99
2012	15.34
2013	15.69
2014	16.01
2015	16.31
2016	16.6
2017	16.87
2018	17.14
2019	17.2