Year	Percent
1990	40.03
1992	45.26
1994	46.68
1995	45.68
1996	48.45
1997	49.53
1998	49.57
1999	50.1
2000	49.25
2001	51.56
2002	52.53
2003	53.04
2004	56.19
2005	57.47
2006	57.99
2007	59.19
2008	58.59
2009	60.97
2010	62.29
2011	63.63
2012	63.79
2013	65.5
2014	66.4
2015	67.18
2016	66.62
2017	68.37