Year	Percent
1991	40.52
1992	40.42
1993	40.39
1994	40.51
1995	40.46
1996	40.47
1997	40.37
1998	40.31
1999	40.23
2000	40.02
2001	39.96
2002	39.9
2003	39.87
2004	39.88
2005	39.99
2006	40.14
2007	40.32
2008	40.5
2009	40.57
2010	40.72
2011	40.9
2012	41.07
2013	41.23
2014	41.42
2015	41.59
2016	41.71
2017	41.82
2018	41.93
2019	42.03
2020	40.56