Year	Percent
1990	18.09
1991	18.31
1992	18.26
1993	18.91
1994	19
1995	19.13
1996	18.91
1997	18.48
1998	17.86
1999	17.61
2000	17.15
2001	16.75
2002	16.39
2003	16.04
2004	15.73
2005	15.43
2006	15.21
2007	15
2008	14.81
2009	14.63
2010	14.45
2011	14.33
2012	14.2
2013	14.07
2014	13.95
2015	15.24
2016	15.96
2017	16.52
2018	16.87
2019	17.16