Year	Percent
1991	43.98
1992	43.95
1993	42.17
1994	41.05
1995	40.41
1996	39.03
1997	38.82
1998	37.66
1999	38.17
2000	38.38
2001	38.56
2002	38.68
2003	38.84
2004	39.4
2005	39.1
2006	38.87
2007	38.56
2008	38.2
2009	38.17
2010	38.36
2011	38.54
2012	38.67
2013	38.76
2014	38.87
2015	38.94
2016	38.96
2017	38.29
2018	37.66
2019	37.04
2020	36.01