Year	Percent
1991	25.84
1992	25.76
1993	23.8
1994	22.56
1995	21.81
1996	20.31
1997	19.99
1998	18.69
1999	19.1
2000	19.24
2001	19.39
2002	19.52
2003	19.73
2004	20.12
2005	20.26
2006	20.41
2007	20.54
2008	20.63
2009	20.68
2010	21.13
2011	21.55
2012	21.91
2013	22.23
2014	22.61
2015	23
2016	23.42
2017	23.12
2018	22.83
2019	22.51