Year	Percent
1991	17.89
1992	17.82
1993	17.67
1994	17.5
1995	17.27
1996	17.02
1997	16.74
1998	16.43
1999	16.1
2000	15.77
2001	15.5
2002	15.21
2003	14.94
2004	14.69
2005	14.52
2006	14.4
2007	14.28
2008	14.14
2009	13.91
2010	13.74
2011	13.57
2012	13.4
2013	13.21
2014	13.06
2015	12.92
2016	12.8
2017	12.68
2018	12.53
2019	12.37