Year	Percent
1991	39.99
1992	40.08
1993	40.05
1994	39.63
1995	39.01
1996	38.42
1997	37.52
1998	36.39
1999	35.32
2000	34.35
2001	34.38
2002	34.65
2003	34.81
2004	34.63
2005	34.02
2006	33.05
2007	31.78
2008	30.5
2009	29.72
2010	28.94
2011	28.77
2012	29.23
2013	29.62
2014	29.66
2015	29.4
2016	29.14
2017	28.87
2018	28.49
2019	28.15