Year	Percent
1991	63.84
1992	64.82
1993	65.66
1994	65.04
1995	65.47
1996	66.42
1997	66.11
1998	64.7
1999	67.92
2000	69.14
2001	69.23
2002	69.97
2003	71.27
2004	70.27
2005	71.53
2006	72.03
2007	72.98
2008	73.29
2009	75.35
2010	75.78
2011	75.78
2012	75.96
2013	76.65
2014	76.87
2015	77.7
2016	78.48
2017	78.58