Year	Percent
1990	23.42
1991	23.38
1992	23.37
1993	23.32
1994	23.27
1995	23
1996	22.97
1997	22.84
1998	22.59
1999	22.31
2000	22.11
2001	21.95
2002	21.88
2003	21.64
2004	21.43
2005	21.23
2006	21.18
2007	21.08
2008	21
2009	20.98
2010	20.9
2011	20.93
2012	20.83
2013	20.78
2014	20.63
2015	20.47
2016	20.31
2017	20.11
2018	19.91
2019	19.72